\int_{0}^{a} f(2a-x)dx\\ let\ u = 2a-x\\ du = -dx\\ when\ x=0,\ u=2a,\ when\ x=a,\ u=a\\ \int_{2a}^{a}f(u)[-du]\\ \int_{a}^{2a}f(u)du\ =\int_{a}^{2a}f(x)dx\\ \int_{0}^{a}f(x)dx+\int_{a}^{2a}f(x)dx\\ =\int_{0}^{2a}f(x)dx
you should start with the LHS side though