We know that:
x = 10 + y
In the next equation {-3x + y = -28} x is the same as the first, so:
-3(10 + y) + y = -28
-30 - 3y + y = -28
-30 - 2y = -28
We add 30 to both sides:
-2y = 2
and divide both sides by (-2):
y = -1
Now we can put y = -1 to the first equation:
x = 10 + y
x = 10 - 1
x = 9
So,
the solution of this system of equations is:
(9, -1)