Guess and check can be a good way to solve a problem like this if you haven't yet studied algebra.
Guess and check can provide good exercise in logical reasoning if you don't just do random guessing. It might look something like this....
x y = 2x+4 x+y
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10 24 34 too small; make x larger
20 44 64 still too small
30 64 94 too big; make x smaller
...
You can continue from there to find the solution.