One side of a rectangle is three times the other. If the perimeter increases by 2%, what is the percentage increase in area?

I've started with these few lines:

Perimeter, P = 2x + 2x + x + x = 8x

Area, 3x^2 = 3(p^2)/64

delta P= 2%

delta A = dA/dP . delta P

delta A = 3/64 (2p). 2%

The answer is 4% but it doesn't look like delta A will be 4%!

Any clues where I went wrong? Thanks.