I have done it with modulus(remainder) function. Modulus of 6^m will be 1 ^m anr that of 4^n is (-1)^n.
The key is to find a relation between m and n. For this question, if you take modulus 5, you get 1m +(-1)n = 1+(-1)n. The first thing to notice is that this is independent of m. Therefore, every value of m is possible. Thus, there are 50 possible m values. To make this equal to 0, you want n to be odd. Thus, there are 25 possible n values. Therefore, there are (50)(25)=1250 number of possible pairs.