Jessica,
I assumed you meant e^x * cos x here and not e^ (x cos x) which would have been a nastier prospect!
In this case you can use the product rule where dy/dx (uv) = u (v') + v (u')
So if u=e^x and v = cos x, u' = e^x and v'=-sin x
and f'(x)=e^x (-sin x) + cos x (e^x)
Hope that helps!
