Question

Find the equation of the parabola with vertex at the origin and directrix is the line $x+3=0$.

Answer 1

Equation of the directrix is
$x+3=0$
$\Rightarrow$ $x=-3$
$\therefore$ $-a=-3$ [Equation of the directrix is $x=-a$]
$\therefore$ equation of the parabola is $y^2=4ax$
$\Rightarrow$ $y^2=4\left(3\right)x$
$\Rightarrow$ $y^2=12x$