Question 917949
When you plugged 6 in for x In the equation {{{7x-6y=-12}}} , you should get
{{{7*6-6y=-12}}}--->{{{42-6y=-12}}}--->{{{42+12-6y=0}}}--->{{{54-6y=0}}}--->{{{54=6y}}}--->{{{54/6=y}}}--->{{{highlight(y=9)}}} .
That checks out, because when you plug {{{x=6}}} and {{{y=9}}} back into the original equations, you get
{{{7*6-6*9=42-54=-12}}} and {{{6*6+7*9=36+63=99}}} .
You got y=-9 because plus and minus signs sometimes turn into each other just to trip students.
It happens to all of us (or at least to most of us).
It did cost me an exam, mostly because instead of moving on to the next problem, I kept working on and reviewing the same problem where I had realized something was off.