Rule of exponents
When you multiply exponents of same base, the exponents add. When you divide exponents of the same base, the exponents substructure
Example: a^x * a^y = a^(x + y)
a^x/a^y = a^(x-y)
x^3 * x^9 = x^(3+9) = x^12
x^15/x^12 = x^(15-12) = x^3
Solution: x^3