Answer
$y^2=-20x$
Work Step by Step
Standard form of a parabola is given as: $y^2=4px$ ...(1)
Formula to find focus is: $F(p,0)$ and formula to find directrix is: $x=-p$ or, $x+p=0$
As we are given that focus is: $(-5,0)$ and directrix is: $x=5$ or $x-5=0$
Plug all these data in equation (1), we have: $p=-5$
Thus, $y^2=4(-5)x \implies y^2=-20x$
