[tex]81x^4-16y^4\implies 3^4x^4-2^4y^4\implies (3x)^4-(2y)^4\implies \underset{\textit{difference of squares}}{[(3x)^2]^2-[(2y)^2]^2} \\\\\\ \underset{\textit{difference of squares}}{[(3x)^2-(2y)^2]}[(3x)^2+(2y)^2]\implies (3x-2y)(3x+2y)[(3x)^2+(2y)^2] \\\\\\ (3x-2y)(3x+2y)[(3^2x^2)+(2^2y^2)]\implies (3x-2y)(3x+2y)(9x^2+4y^2)[/tex]
