First, the hours will be x and the temperature will be y. So we start with having (4, -13). A "steady rate" is our slope in these kinds of problems, so m=5.
For this problem you use slope-form, or y=mx+b . To find b you plug in the given information (4, -13) and m=5, to get b= -33 .
Your equation is then y= 5x-33 .
Plug in 12 for y, solve for x and you will get x= 9.
Or in other words, at 9 am the temperature will be 12 degrees.
