$ 2x^2+6x+11 $
$ 2\left(x^2+3x+\frac{11}{2}\right) $
$ 2n=3 \implies n=\frac{3}{2} $ which you want to square $ \implies \left(\frac{3}{2}\right)^2 $
$ 2\left(x^2+3x+\frac{11}{2}+\left(\frac{3}{2}\right)^2\right) =2 \left( \left(\frac{3}{2}\right)^2 \right) $
$ 2\left(x^2+3x+\frac{11}{2}+\left(\frac{3}{2}\right)^2-\left(\frac{3}{2}\right)^2\right) $
completing the square
$ 2\left(\left(x+\frac{3}{2}\right)^2+\frac{11}{2}-\left(\frac{3}{2}\right)^2\right) \implies 2\left(\left(x+\frac{3}{2}\right)^2\right) + 2\left( \frac{11}{2}-\frac{9}{4} \right) $
$\Updownarrow $
$ 2\left(x+\frac{3}{2}\right)^2+\frac{26}{4} \equiv a(x - h)^2 + k $
$ \overset{. \: .}{\smile } $
.