You are certainly on the right track! However, do not forget that the first derivative of a function is simply the "slope" of the original function.
To find out when the original function will have a slope of 5, simply set the first derivative (which you have already determined) equal to 5 and solve for x.
You will get "two" solutions for x since you are solving a quadratic equation. Once you find each x value, simply plug them back into the original cubic equation to find the matching y coordinate.
Hope this helped