Let \( \Delta \) be the area of the region \( \{(x,y)\in \mathbb{R}^{2}: x^{2}+y^{2}\leq 21,\; y^{2}\leq 4x,\; x\geq 1\} \). Then \( \dfrac{1}{2}\!\left(\Delta - 21\sin^{-1}\!\dfrac{2}{\sqrt{7}}\right) \) is equal to:
\( \sqrt{3}-\dfrac{4}{3} \)
\( \sqrt{3}-\dfrac{2}{3} \)
\( 2\sqrt{3}-\dfrac{2}{3} \)
\( 2\sqrt{3}-\dfrac{1}{3} \)