## Precalculus (6th Edition) Blitzer

$y^2=-40x$
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, $y^2=4(-10)x \implies y^2=-40x$