\(a,=3\left(x+y\right)+\left(x-y\right)\left(x+y\right)\\ =\left(x+y\right)\left(3+x-y\right)\\ \\ d,=\left(x^2-y^2\right)\left(x^2+y^2\right)\\ =\left(x-y\right)\left(x+y\right)\left(x^2+y^2\right)\)
c)(3x+3y)+(x2-y2)=3(x+y)+(x-y)(x+y)=(x+y)(3+x-y)
d)x4-y4=(x2-y2)(x2+y2)=(x-y)(x+y)(x2+y2)