Substitute x = 2y+6 into the first equation. Solve for y. Then substitute y into either equation (preferably the second) to find x.
Generally, I don't post entire solutions as this is a homework help forum. I can't just do the whole problem for you, you won't learn as much. I'll help you get started:
We substitute x = 2y+6 into the first equation:
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Expand:
You should be able to simplify the above expression into a quadratic in terms of y. Hopefully you know how to solve quadratics. If you don't, you should review that before continuing on with the problem.