Explanation:
The rule used is
a^b*a^c = a^(b+c)
Basically you add the exponents. This only applies if the bases are both the same. In this case, they're both 4 so the rule works out.
Using the rule, we get
a^b*a^c = a^(b+c)
4^5*4^10 = 4^(5+10)
4^5*4^10 = 4^15
