if f={(2 3), (4 9), (3 -8)} and g={(1 2), (2 5), (3 10), (4 17)} then find gf

Answer 1

if by gf you mean g*f, then just multiply the y-values for all pairs which share a common x-value
g*f = {(2,15),(3,-80),(4,153)} since f(1) is undefined.