Double integral of exponential function [closed]

Evaluate $$\int^{+\infty}_{-\infty}\, \int^{+\infty}_{-\infty}\, e^{-(3x^{2}+2\sqrt{2}xy+3y^2)}dxdy\,.$$

closed as off-topic by John B, G Cab, José Carlos Santos, mlc, Ethan BolkerJan 17 '18 at 19:44

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$$\int_{-\infty}^\infty\exp\left(-\text{n}\cdot x^2-\text{m}\cdot x\right)\space\text{d}x=\frac{\sqrt{\pi}}{\sqrt{\text{n}}}\cdot\exp\left(\frac{\text{m}^2}{4\cdot\text{n}}\right)\tag1$$
When $\Re\left(\text{n}\right)>0$
$$\int^{+\infty}_{-\infty}\, \int^{+\infty}_{-\infty}\, e^{-(3x^{2}+2\sqrt{2}xy+3y^2)}dxdy=\int^{+\infty}_{-\infty}e^{-3y^2}\, \int^{+\infty}_{-\infty}\, e^{-3x^{2}-2\sqrt{2}xy}dxdy\tag2$$