Remember that you can add the exponents of in a product of terms
with the same base, so mx * m7 = mx+7
= m28, so that
x + 7 = 28, making x = 21. Remember also that in an exponent
raised to another exponent, you can multiply those exponents. So
(m5)y becomes m5y, and 5y = 15, so y is
3. x + y is 21+3 = 24, which is choice D.