The rule for making a negative exponent positive
if it is a factor of either the numerator of the denominator:

1.  If the base with a negative exponent is a factor of the
NUMERATOR, move the base and exponent as a factor of the
DENOMINATOR and change the sign of the exponent to positive.

2.  If the base with a negative exponent is a factor of the
DENOMINATOR, move the base and exponent as a factor of the
NUMERATOR and change the sign of the exponent to positive.

Yours is case 2, so bring the factor  from the
DENOMINATOR to the NUMERATOR and change the exponent "-3"
to "3", like this:



Then you can dispense with the 1:



[Note: Be sure not to bring the 3 to the numerator with the
x³ because the 3 already has an understood positive exponent
1, since 3 = 31.] 

Edwin