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
