The sum of two numbers is - 12 their difference is 84 find the two numbers

Assume that the numbers are X and Y;

The sum of the two numbers is:

X+Y=-12

Their difference is:

X-Y=84

We can present the number X as a funsction from Y, from any of the equation. Let do that from the first: X=-12-Y

Now, in order to have only one unknown we will write X in this form in the second equation:

(-12-Y) - Y=84

-12-Y-Y=84

-12-2Y=84

-12-84=2Y

-96=2Y

Y=-96/2=-48

X=-12-Y=-12 - (-48) = - 12+48=36