a, 5xy(x+3y)
b, (x-1)(3+5x)
c, (x-y+3)(x-y-3)
a)\(5x^2y+15xy^2=5xy\left(x+y\right)\)
b) \(3x-3+5x\left(x-1\right)=3\left(x-1\right)+5x\left(x-1\right)=\left(x-1\right)\left(3-5x\right)\)
c)\(x^2-2xy-9+y^2=\left(x^2-2xy+y^2-9\right)=\left(x-1\right)^2-9=\left(x-1-9\right)\left(x-1+9\right)=\left(x-10\right)\left(x+8\right)\)