Given the vertex of the parabola is $(-3,0)$
The equation of directrix is $x+5=0$
Equation of the parabola is $(y-0)^2=4a(x-(3))$
(ie) $y^2=4a(x+3)$
$x=-a$ is the equation of the directrix .
Given directrix is $x+5$
(ie) $x+5-3=0$
(ie) $x+2=0$
$\therefore a=2$
$\therefore$ Equation of the required parabola is $y^2=8(x+3)$