There are two ways:
One if you have a graphing calculator (TI-84 plus) you can go to stat then edit then type in the -40 and -90 on the left and the -120 and 60 on the right. The press stat again and go to the right to calc the down to linreg (ax +b) and go down to calculate amd press enter.
Your answer is y = -3.6x+-264
option two is by hand
the formula is y1-y2 divided by x1-x2
