$a)16{\left(x+\frac{1}{2}y\right)}^2{\left(x-\frac{1}{2}y\right)}^2={\left(4\left(x+\frac{1}{2}y\right)\left(x-\frac{1}{2}y\right)\right)}^2={\left(4\left(x^2-\frac{1}{4}{y}^2\right)\right)}^2={\left(4x^2-y^2\right)}^2=16x^4-8x^2{y}^2+y^2$