Can someone please help me with this question, many thanks.

Suppose that f is a 2-variable real-valued function defined on a rectangle D, that is, f : [a, b] x [c, d] - R, with D = [a, b] x [c, d]. Also suppose that D' is another rectangle that is a subset of D, so that D' = [a', b'] x [c', d'] with a