Hi, I have a problem but I can't figure out what the equation should be, (like what numbers I should be using)

A square exercise pen measuring 6 feet on each side needs to be expanded as the golden retriever pups grow bigger. By how much must each side be lengthened to increase the area to more than double the original area? The shape must be kept a square.

I've tried A= x^2 and x=6+y but that doesn't work :/

thanks!