you want to convert the integral from integral(f(x)dx) to integral(f(u)du)
if u=x^2-6x, then u'(x)=du/dx=2(x-3).
thus du=dx u'(x)=2(x-3)dx
thus -1/2*du=(3-x)dx
the original integral is integrate( (3-x)(x^2-6x)^5 dx)
=integrate( u^5 (3-x)dx)
=integrate ( u^5*(-1/2) du)
after integrating, convert back to x using the fact that u=x^2-6x