Math Help - Double Integral Question

1. Double Integral Question

How do I show:
(see attachment)

by changing the order of integration?

Thanks

2. $\int_{0}^{a}{\int_{0}^{x}{f(y)\,dy}\,dx}=\int_{0}^ {a}{\int_{y}^{a}{f(y)\,dx}\,dy},$ and we're done.