The directrix of the parabola -40x = y2 is:

The directrix of parabola is the line  x = +10.

Step-by-step explanation:

The general form of parabola is given as

where the directrix  is the vertical line x  =  - p .

If p > 0, then parabola opens to the right.

If p < 0 then parabola opens to the left.

Now here, the given equation is

Representing the given equation in standard form:

⇒  p  = -10

So, the directrix of the parabola  is x  = - p  = - (-10)  = 10

or, x  = + 10

Hence, the directrix of parabola is the line  x = +10.