In a fraction, the number below the line (called the denominator) represent the whole. In this problem '8' is in the denominator and represents the 8 slices of pizza there were to start with
Answer=8 slices
= x1 + x2/2 , y1 + y2/2
= 7+-1/2 , 1+(-1)/2
Midpoint = (3,0)