You can put this solution on YOUR website! Absolute value simply means the magnitude of a quantity: its distance from zero. It is the same as asking "How far away is n away from 0?" Where n is any real number.
Moreover, Abs(-a)=Abs(a)=a.
I will allow you to solve the problem for -31, but will offer a few examples as follows:
The absolute value of -880 is 880.
The absolute value of 880 is 880.
The absolute value of -4 is 4.
The absolute value of 4 is 4.