Remark
Any two points are enough to solve what you need. You are going to have to make some assumption about how to number the squares. Let us say that for each square, the x value increases by 1 in the x direction and 1 in the y direction.
Givens
One point is (2,1) The other is (0, -3)
Solution
m = (y2 - y1) / (x2 - x1)
m = (1 - - 3) / (2 - 0)
m = ( 1 + 3)/2
m = 2
The y intercept can be solved by using either of the 2 given point. Most people would pick (0,-3) but let's see if (2,1) works.
So far what you have is
y = 2x + b
x = 2
y = 1
1 = 2(2) + b
b = 1 - 4
b = - 3 Just as it would have if you used (0,-3)
Answer: y = 2x - 3
