Answer
$x^2=12y$, $3$ inches above the vertex.
Work Step by Step
Step 1. Use the figure given in the exercise, we can write the general form of the parabola as $x^2=4py$
Step 2. As point $(6,3)$ is on the parabola, we have $6^2=4p(3)$ which gives $p=3$. Thus the parabola is $x^2=12y$
Step 3. The light the source should be placed at the focus which is $3$ inches above the vertex.
