The additive inverse of a number is a number when added to the original number equals to 0.
What can we add to 51 to make it equal to 0. Set up an equation.
Let x = the additive inverse of 51
x + 51 = 0
Solve for x.
x + 51 = 0
x = -51 <-- Subtract 51 from both sides
So, -51 is the additive inverse of 51.
