Idon't understand why i got it wrong(

You got the answer y=78 by substituting x=20 into the top angle, 3x+18.
But the angle 3x+18 is not equal to the angle with the y variable.
They are supplementary angles (equal 180) but are not necessarily equal.
The angle that equals 2(y+6) is the angle opposite to it, 5x+2 (Vertical angle theorem).
So you should have substituted 20 into 5x+2