Answer
$y^2=-16x$
Work Step by Step
Directrix: $x=4$
In a parabola with horizontal axis:
Directrix: $x=-p$
So, $-p=4$
$p=-4$
Equation of a parabola with horizontal axis: $y^2=4px$
$y^2=4(-4)x$
$y^2=-16x$
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