The expression is given as (-3)^2
This can be written in multiplication form by spliting the number into two factors, but the exponent remains unchanged. Hence, -3 can be split into
-3 and 1, hence
-3^2 * 1^2 = (-3 * 1)^2
-3^2 * 1^2 = (-3)^2
Basically when two numbers have the same index power, then the product of both numbers would be raised to the power associated with each factor.
Since -3 and 1 both are raised to the power of 2, then their product would be raised to the power of 2.