Evaluate the indefinite integrals using substitution. (Use $C$ for the constant of integration.) a) $\displaystyle \int {3x^{2}\!\left(x^{3}-6\right)^{6}} \, dx =$ b) $\displaystyle \int {\left(2x+3\right)\!\left(x^{2}+3x+9\right)^{6}} \, dx =$ c) $\displaystyle \int {x\!\left(x^{2}-5\right)^{7}} \, dx =$ d) $\displaystyle \int {\left(20x+10\right)\!\left(5x^{2}+5x+5\right)^{5}} \, dx =$