#NUM! problem

Using the Evaluate formula tool (https://support.office.com/en-us/art...6-a70aa409b8a7 ), I see that your formula in M13 is trying to take a negative number to a fractional power. You probably need to spend some time trying to decide if the "base" in this problem should be negative. If it is not, figure out what is wrong that is causing that part of the formula to be negative. If it is, then try to decide what (-1)^(5/6) means in the context of your problem. Then adjust the formula accordingly.