could you explain why the answer 125 was supposed to be negative while my answer 81 was correct?

Answer:
Step-by-step explanation:
Realize what (-5)^3 means
(-5)(-5)(-5)
25(-5)
-125
meanwhile
(-9)^2 is
(-9)(-9)
81
Basically a negative times a negative is always positive, but when a negative number is cubed (or raised to any odd exponent for that matter) the product will be negative.