Here the number is negative.

So during multiplication care has to be taken of the sign.

The basic rules are:

Negative Number × Negative Number = Positive

Negative Number × Positive Number = Negative Number

Positive Number × Positive Number = Positive Number

The cube of -5 can be written as follows:

⟹ -5 × -5 × -5

⟹ 25 × -5

= -125

Hence (-5)³ = -125