ANSWER
p=-1
EXPLANATION
The given equation is
[tex] {y}^{2} = - 4x[/tex]
We compare with the general equation of the parabola with vertex at the origin.
[tex]{y}^{2} =4px[/tex]
Comparing the right hand side we have,
[tex]4px = - 4x[/tex]
Divide both sides by 4x
[tex] \frac{4px}{4x} = \frac{ - 4x}{4x} [/tex]
This implies that,
[tex]p = - 1[/tex]
