$P=\frac{\left(b+4a+b\right)\cdot 2b}{2}=\left(4a+2b\right)\cdot b=4ab\cdot 2b^2=\frac{6}{6}\cdot 4ab\cdot 2b^2=\frac{1}{6}\cdot \left(24ab+12b^2\right)$