If f(x)=-2x³+x²+16x-15 and g(x)=x+3, we can substitute the polynomial in for f(x) and the binomial for g(x) like so:

The next step is to factor the numerator by grouping in the hopes that something will cancel!
Unfortunately, because we can't factor anything out of the second grouped pair, that means there's no simplifying possible! So...

Hope this helps!