Question 1114351
Vertex as the origin, concave upward.
{{{y=ax^2}}}



Two points other than (0,0) are (-18,3) and (18,3), unit being feet.



{{{a=y/x^2}}}

{{{a=3/(18^2)}}}

{{{a=3/(3*6*18)}}}

{{{a=1/(6*18)}}}

-

{{{y=(1/108)x^2}}}

OR equivalently

{{{highlight_green(108y=x^2)}}}



Let p be distance above the vertex to put the receiver  (placement of the focus).


{{{4p=108}}}

{{{p=108/4}}}

{{{highlight(p=27)}}}