\[ \boxed { {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) =1/3\,x \left ( -2+3\,\sqrt {{x}^{2}+3\,y \left ( x \right ) } \right ) =0} \]
Mathematica: cpu = 0.085511 (sec), leaf count = 33 \[ \left \{\left \{y(x)\to \frac {1}{48} \left (-54 c_1 x^2+81 c_1^2+9 x^4-16 x^2\right )\right \}\right \} \]
Maple: cpu = 0.156 (sec), leaf count = 23 \[ \left \{ {\it \_C1}+{\frac {3\,{x}^{2}}{4}}+{\frac {2}{3}}-\sqrt {{x}^ {2}+3\,y \left ( x \right ) }=0 \right \} \]