a) \(x\left(y-1\right)+3\left(y-1\right)=\left(x+3\right)\left(y-1\right)\)
b) \(x^2-2xy+y^2-9\)
\(=\left(x^2-2xy+y^2\right)-3^2\)
\(=\left(x-y\right)^2-3^2\)
\(=\left(x-y-3\right)\left(x-y+3\right)\)
a) x(y-1) + 3(y-1) = (x+3)(y-1)
b) x\(^2\) - 2xy + y\(^2\) - 9 = (x-y)\(^2\) - 3\(^2\) = (x-y-3)(x-y+3)