In this question, you have two variable p and q. To simplify it, you need to divide each variable which will result in subtracting the power/exponent. Since all the exponent is a minus, it will be easier to switch the position first. The calculation should be:
15 * p^-4 * q^-6 / -20 * p^-12 * q^-3=
15 * p^12 * q^3 / -20 * p^4 * q^6=
15 /-20 * p^(12-4) * q^(3-6)=
-3 /4 * p^8 * q *-3=
-3 * p^8/ q^3