Let $\Delta$ be the area of the region $\{(x,y)\in\mathbb{R}^{2}:\ x^{2}+y^{2}\le 21,\ y^{2}\le 4x,\ x\ge 1\}$. Then $\dfrac{1}{2}\Big(\Delta-21\sin^{-1}\!\dfrac{2}{\sqrt{7}}\Big)$ is equal to: