I need to integrate (x^4)(e^-x) but am not sure what rule it is.
Thanks.
Can you show some work? Proably you'll need integration by parts 4 times.
We have:
$\displaystyle \int x^4e^{-x}dx=\int x^4d(-e^{-x})=-x^4e^{-x}+\int e^{-x}4x^3dx=-x^4e^{-x}+4\int e^{-x}x^3dx$
Now you have to apply integration by parts again to calculate:
$\displaystyle \int e^{-x}x^3dx$
etc ...