Answer:
If you look over the steps you can see that until 4x + x + 3 = 18, evertything is dandy. But the step after that 4x + x =21 seems a bit fishy.
Think about it they subtract 3 from both sides so the first side is correct
4x + x, but they added 3 to the other side:
not
4x+x = 21
Then we solve for 4x + x = 15
To solve for y we use :
y = x+3
y = 3+3 = 6
so (3,6) is the right answer