int x^2 * a^x dx = int x^2 d(a^x/ln a) = x^2 * a^x/ln a - int a^x/ln a d(x^2) =
= 1/ln a * x^2 * a^x - 2/ln a * int x * a^x dx =
= 1/ln a * x^2 * a^x - 2/ln a * int x d(a^x/ln a) =
= 1/ln a * x^2 * a^x - 2/ln a * x * a^x/ln a + 2/ln a * int a^x/ln a dx =
= 1/ln a * x^2 * a^x - 2/ln^2 a * x * a^x + 2/ln^2 a * int a^x dx =
= 1/ln a * x^2 * a^x - 2/ln^2 a * x * a^x + 2/ln^3 a * a^x + C