d: \(\left(xy+4\right)-\left(2x+2y\right)^2\)
\(=\left(xy+4-2x-2y\right)\left(xy+4+2x+2y\right)\)
\(=\left[x\left(y-2\right)-2\left(y-2\right)\right]\left[x\left(y+2\right)+2\left(y+2\right)\right]\)
\(=\left(y-2\right)\left(x-2\right)\left(y+2\right)\left(x+2\right)\)
f: \(x^2-4xy+3y^2\)
\(=x^2-xy-3xy+3y^2\)
\(=x\left(x-y\right)-3y\left(x-y\right)\)
\(=\left(x-y\right)\left(x-3y\right)\)