Question 19555
First, get the equation in terms of y:

{{{x = -2y^2}}} Divide both sides by -2.
{{{x/-2 = y^2}}} Now take the square root of both sides.
{{{y = +sqrt(x/-2)}}}, {{{y = -sqrt(x/-2)}}}

Take a look at the graph:

{{{graph(300,200,-5,5,-3,3,sqrt(x/-2),-sqrt(x/-2))}}}