I think the trick here is that the information “m and n are greater than 3” is not actually as important. Basically, we need to find the highest possible powers in the denominator such that the result remains an integer. Hence we need to find out the number of factors of the numerator, which is (n+1)(m+1).
We then multiply this result by 2 to consider the possibilities of negative integers. Since for every positive integer factor, we have a negative integer factor, we consider twice the above result.
Some more discussion on this problem here -