Is it possible to solve this?

I feel kinda dumb for not knowing what to do here, but I know my math knowledge is somewhat lacking, and I figure people here know what they are doing, so..

This isn't homework or anything, I just wrote random numbers down on a dry erase board and tried to find an answer. I'm looking for the value of y.

(7x-3y)^2-6 = 7

So far I've done..

49x^2-42xy+9y^2 = 13
49x^2-y(42x)+y(9y)-13 = 0
49x^2-13 = y(42x)-y(9y)

But I'm not sure where to go from here, or if it's even possible to.

Thanks for the help.