Circle the first line.
His mistake was: He substituted y from 2nd equation in 1st equation incorrectly. There is a minus sign before y and it wasn't taken into the account.
The correct steps are:
2x - y = 18
2x - (4x + 2) = 18
2x - 4x - 2 = 18
-2x - 2 = 18
-2x = 20
x = -10
y = 4x + 2
y = 4(-10) + 2
y = -40 + 2
y = -38
