Question

What is the equation of the line passing through the points (-1,2) and (-2,-1)

Answer 1

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.