I don't understand how these are the correct answers. can someone please show work to explain how it is done?

b gtb) Use the substitution formula If(g(x))g'(x)dx = J- f(u)du, where g(x) = u, to evaluate the following integrals. 3 9(3) 5 2 (a) Irx/25-r2dr (b) I n/25r2 dr 0 -2 5 125 (a) IN 25 - r2 dr = T (Simplify your answer. Type an integer or a simplied fraction.) 0 2 (b) I rd 25 - r2 dr= 0 (Simplify your answer. Type an integer or a simplied fraction.) - 2