Answer
$\frac{3}{4}$in from the vertex.
Work Step by Step
Step 1. Setup a xy-coordinate system so that the origin is at the vertex of the parabola.
Step 2. Simulate the reflector with an equation $y^2=4px$
Step 3. At $x=3$, we have $y=6/2=3$, plug them in the equation to get $3^2=4p(3)$, thus $p=\frac{3}{4}$
Step 4. The filament is at the focus which is given by $F(p,0)$ or $F(\frac{3}{4},0)$ which means that the filament should be place $\frac{3}{4}$ in from the vertex.
