Hi there!
So, our two equations are:
2x + 3y = 20 and
-2x + y = 4
We can see that the x's will cancel out because they're the same number, opposite signs. Then we're left with 4y = 24.
Divide 24 by 4, which is 6.
y = 6, then we plug that in to the first equation for y:
2x + 3(6) = 20
2x + 18 = 20
2x = 2
x = 1
So, she made her first mistake when adding the equations, adding 20 and 4, she somehow got 16.
The solution to the system is (1,6).
I hope I helped!
