Complete the square; factor out the leading coefficient first. For example, in $$3x^2 + 3x + \underline{\phantom{aaa}} = 3(x+\underline{\phantom{aaa}})^2$$ fill in $\frac{3}{4}$ and $\frac{1}{2}$ $$3x^2+3x + \frac{3}{4} = 3(x^2 + x +\frac{1}{4})= 3(x+\frac{1}{2})^2.$$
$2 x^2 - 4 x +$ $= 2 (x -$ $)^2$.
$3 x^2 + 6 x +$ $= 3 (x +$ $)^2$.