[tex]2x^2+4=20\ \ \ \ |subtract\ 4\ from\ both\ sides\\\\2x^2=16\ \ \ \ \ |divide\ both\ sides\ by\ 2\\\\x^2=8\iff x=\pm\sqrt8\\----------------------------\\\sqrt8=\sqrt{4\cdot2}=\sqrt4\cdot\sqrt2=2\cdot\sqrt2=2\sqrt2\ \ \ |use\ \sqrt{a\cdot b}=\sqrt{a}\cdot\sqrt{b}\\-----------------------------\\therefore\ your\ answer\ is\ -2\sqrt2\ and\ 2\sqrt2\to\boxed{a\ and\ c}[/tex]
