Ok, so first this is a system. You have:
8x+12y=3200
And
X+y=300.
That means y= -X + 300.
If you replace "y" in the first equation with -X+300, it would be 8x+12(-X+300)=3200.
If you simplify that, X will be equal to 100. Now go back to the second equation, which is X+y= 300.
Now that you know X is 100, you can subtract it from 300 to find y.
So,
X=100 and y=200!
Hope this helps!!!