I’m going to start you off with a hint instead of giving you the full solution.

What you may notice is that both equations contain + 3y.

Set the two equations up such that one of them stays the same,and in the other one, each term, on both sides of the equal sign, is multiplied by negative one (or as Sal Khan often says in videos here, incorrectly according to the teachers I had when I was growing up, “minus one”).

So you will have either (read it as “ee-ther” when I am the communicator):

x + 3y = 7

-5x - 3y = 1

or

-x - 3y = -7

5x + 3y = -1

The next step is to subtract one of the equations from the other, on both sides of the equal sign. This will eliminate the 3y term. You will get “ee-ther”:

-4x = 8 or 4x = -8

Solve for x and then substitute it in one of the original equations to solve for y.