Answer
$y^2=-20x$
Work Step by Step
Focus: $(-5,0)$; Directrix: $(x=5)$
$y^2=4px$ (parabola equation)
$F(p,0)$ (focus formula)
$x=-p$ (directrix formula)
$F(-5,0)$
$4(-5)=-20$
$y^2=-20x$
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