Given equation is $y^2 = 16\,x$
Here, $\,\,\,\,\,\,a = 4$
As we know, that if tangents are drawn from an external points of a focal chord, then it will intersect on the directrix of the parabola.
$\therefore\,\,\,\,\, $ Equation of directrix is $x = - 4$
or $\,\,\,\,\,\,x + 4 = 0 $
Which is the required equation.