Find the equation (in terms of x) of the line through the points (-2,-1) and (1,-4)

First we find the slope. 

Slope or m = 
where X1 is the x-coordinate of the first ordered pair and Y1 is the y-coordinate of the first ordered pair. Same applies to X2 and Y2 for the second ordered pair.

m = 
m = 
m = 
m = - 1

Your equation starts as y = - 1x + b or just y = - x + b

Now we plug either ordered pair into the equation and solve for b to get our complete linear function.

- 4 = - (1) + b
- 4 = - 1 + b
- 4 + 1 = b
- 3 = b

Your equation is:

y = - x - 3