so (1-1)/(-8-7) = 0/15 so the slope is zero meaning it is a horizontal line which we can tell logically because the y coordinates of both points are 1.
so if you plug those numbers into the formula y=mx+b where m is slope and b is the y intercept then
you get y=0*x+1
then 0*x is just zero and it goes away so the equation for this line is y=1