\(x^4+4=x^4+4x^2+4-4x^2=\left(x^2+2\right)^2-\left(2x\right)^2=\left(x^2-2x+2\right)\left(x^2+2x+2\right)\)
\(x^2+2xy+y^2-16=\left(x+y\right)^2-4^2=\left(x+y-4\right)\left(x+y+4\right)\)
\(3x^2+5x-3xy-5y=3x\left(x-y\right)+5\left(x-y\right)=\left(x-y\right)\left(3x+5\right)\)