(х+4у)^2 - (4у-х)(х+4у)

(х+4у)^2 - (4y-x)(4y+x) = (x^2+8xy+16y^2) - (16y^2-x^2) = (x^2+8xy+16y^2) - -16y^2+x^2 = x^2+x^2+8xy = 2x^2+8xy