a)\(7xy-7x^2=7x\left(y-x\right)\)
b)\(x^2+14xy+49y^2=\left(x+7y\right)^2\)
c)\(3x^2-3y^2-5x+5y=3\left(x^2-y^2\right)-5\left(x-y\right)=3\left(x+y\right)\left(x-y\right)-5\left(x-y\right)=\left(x-y\right)\left(3x+3y-5\right)\)
d)\(3x+3y-x^2-2xy-y^2=3\left(x+y\right)-\left(x+y\right)^2=\left(3-x-y\right)\left(x+y\right)\)