Take 2 points
(3, 9) and (0, 3)
Slope = (Y2 -Y1) ÷ (X2 -X1)
Slope = (3-9) / (0-3)
Slope = -6 / -3
Slope = 2
The equation will have the form y = mx + b
We can rearrange this to b = y -mx
Let's take one of the points (3, 9) and the slope (m) and enter it into the formula:
b = -9 -2*3
b= -3
Now we enter all that data into b = y -mx
and get
y = 2x -3