$a=-\sqrt{b^{2}-3x}$
$a=\sqrt{b^{2}-3x}$
$a=\sqrt{3x-b^{2}}$
$a=-\sqrt{3x-b^{2}}$

$b=\sqrt{3x-a^{2}}$
$b=-\sqrt{3x-a^{2}}$
$b=-\sqrt{3x+a^{2}}$
$b=\sqrt{3x+a^{2}}$

$\left\{\begin{matrix}a=\sqrt{b^{2}-3x}\text{; }a=-\sqrt{b^{2}-3x}\text{, }&x\leq \frac{b^{2}}{3}\\a=\sqrt{3x-b^{2}}\text{; }a=-\sqrt{3x-b^{2}}\text{, }&x\geq \frac{b^{2}}{3}\end{matrix}\right.$

$\left\{\begin{matrix}b=-\sqrt{3x-a^{2}}\text{; }b=\sqrt{3x-a^{2}}\text{, }&x\geq \frac{a^{2}}{3}\\b=-\sqrt{3x+a^{2}}\text{; }b=\sqrt{3x+a^{2}}\text{, }&x\geq -\frac{a^{2}}{3}\end{matrix}\right.$