\(a,=\dfrac{12x^2y\left(x+y\right)^2:3x\left(x+y\right)}{3x\left(x+y\right):3x\left(x+y\right)}=4xy\left(x+y\right)\\ b,=\dfrac{x^2+6x+9+24-x^2+4x-3}{4\left(x-3\right)\left(x+3\right)}=\dfrac{10\left(x+3\right)}{4\left(x-3\right)\left(x+3\right)}=\dfrac{4}{2\left(x-3\right)}\)