The error carolyn has made is that she hasn't subtracted properly.
2x + 3y - (2x +6y) = 2x - 2x + 3y - 6y = -3y.
Carolyn probably didn't use parentheses...
To do the thing in substitution, we single out x or y in any equation. In this case, i'm gonna single out the x in the 2nd eqn. In other words, i'll make x the subject of the 2nd eqn.
2x + 6y = 10
2x = 10 - 6y
x = 5 - 3y
Now we simply plug in 5 - 3y wherever x apperas in the 1st equation.
