Find two numbers whosse sum is 12 and whose difference is 4

X + y = 12

x - y = 4

From the second equation we can see that x = 4 + y

We can plug this into the first one and get:

4 + y + y = 12

4 + 2y = 12

2y = 12-4

2y = 8

y = 4

And then we can plug that into "x" and get that x = 4 + 4, or x=8