please help me simplify this expression.

Another way to think about it, and yet another rule of exponents, is that x

^{a}*x^{b}=x^{a+b}(it's the flipped version of what Phillip had said).Thus 4

^{3}*4^{-5}=4^{3+(-5)}=4^{3-5}=4^{-2}.