giai cac he pt sau
a/ $\left\{ \begin{array}{l} 2( x + y) + 3xy= 12\\ x^2 + y^2 + xy=7 \end{array} \right.$
b/ $\left\{ \begin{array}{l} \sqrt{x} + \sqrt{y} = 9\\ \sqrt[3]{x} + \sqrt[3]{y} = 5 \end{array} \right.$
c/ $\left\{ \begin{array}{l} \sqrt{x} + 2\sqrt[4]{y} = 1\\ x + 2y =1 \end{array} \right.$