you use the change in Y over the change in X ex: y-y/x-x in your case it would be -8-(-4)/-2-(-9) that would give you -4/-7 or 4/7 that would be M in the formula Y=mx+b. to find B you would plug in one of the point values and M into the formula. -8=4/7(-2)+b that would equal -8=-8/7+b. then you add 8/7 to both sides of the equation. that would give you b=-6 6/7 so if you plug m and b in to the equation into the formula you get y=4/7x-6 6/7.