The equation will be of the form y=ax+b, but we have to find a and b.
If you fill in the first point, you get:
2 = -a + b => b = 2+a
Second point:
-1 = -2a + b
Fill in first into second
-1 = -2a + 2 + a => -3 = -a => a=3
b = 2+a = 2+3 = 5
so the equation is
y=3x+5
Once you have this, it is safe to fill in both coordinates to double-check for mistakes.