Answer
$y^2=-40x$
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: $(-10,0)$ and directrix is: $x=10$ or $x-10=0$
Plug all these data in equation (1), we have: $p=-10$
Thus, $y^2=4(-10)x \implies y^2=-40x$
