Equation of a Straight Line
The equation of a straight line is usually written this way:
(or "y = mx + c" in the UK see below)
What does it stand for?
Slope (or Gradient)  Y Intercept 
y = how far up
x = how far along
m = Slope or Gradient (how steep the line is)
b = the Y Intercept (where the line crosses the Y axis)
How do you find "m" and "b"?
 b is easy: just see where the line crosses the Y axis.
 m (the Slope) needs some calculation:
m = 

Knowing this we can work out the equation of a straight line:
Example 1
m  = 

=  2 
b = 1 (where the line crosses the YAxis)
So: y = 2x + 1 
With that equation you can now ...
... choose any value for x and find the matching value for y
For example, when x is 1:
y = 2×1 + 1 = 3
Check for yourself that x=1 and y=3 is actually on the line.
Or we could choose another value for x, such as 7:
y = 2×7 + 1 = 15
And so when x=7 you will have y=15
Example 2
m  = 

=  −3 
b = 0
This gives us y = −3x + 0
We do not need the zero!
So: y = −3x 
Example 3: Vertical Line
What is the equation for a vertical line?
The slope is undefined ... and where does it cross the YAxis?
In fact, this is a special case, and you use a different equation, not "y=...", but instead you use "x=...".
Like this:
x = 1.5 
Every point on the line has x coordinate 1.5,
that is why its equation is x = 1.5
Rise and Run
Sometimes the words "rise" and "run" are used.
And so the slope "m" is:
You might find that easier to remember 
Now Play With The Graph !You can see the effect of different values of m (the slope) and b (the y intercept) at Explore the Straight Line Graph 
Other Forms
We have been looking at the "slopeintercept" form. The equation of a straight line can be written in many other ways.
Another popular form is the PointSlope Equation of a Straight Line.
Footnote
Country Note:
Different Countries teach different "notation" (as sent to me by kind readers):
In the US, Australia, Canada, Eritrae, Iran, Mexico, Portugal, Philippines and Saudi Arabia the notation is:  y = mx + b 
In the UK, Australia (also), Bahamas, Bangladesh, Belgium, Brunei, Bulgaria, Cyprus, Egypt, Germany, Ghana, India, Indonesia, Ireland, Jamaica, Kenya, Kuwait, Malaysia, Malawi, Malta, Nepal, Netherlands, New Zealand, Nigeria, Oman, Pakistan, Peru, Poland, Singapore, Solomon Islands, South Africa, Sri Lanka, Turkey, UAE, Zambia and Zimbabwe  y = mx + c 
In Afghanistan, Albania, Brazil, China, Czech Republic, Denmark, Ethiopia, France, Lebanon, Holland, Kosovo, Kyrgyzstan, Norway, Romania, South Korea, Spain, Tunisia and Viet Nam:  y = ax + b 
In Azerbaijan, China, Finland, Russia and Ukraine:  y = kx + b 
In Greece:  ψ = αχ + β 
In Italy:  y = mx + q 
In Japan:  y = mx + d 
In Cuba and Israel:  y = mx + n 
In Latvia:  y = jx + t 
In Romania:  y = gA + C 
In Sweden:  y = kx + m 
In Serbia and Slovenia:  y = kx + n 
In your country:  let us know! 
... but it all means the same thing, just different letters.