After a dreary day of rain, the sun peeks through the clouds and a rainbow forms. you notice the rainbow is the shape of a parabola.

the equation for this parabola is y = -x2 + 36.

graph of a parabola opening down at the vertex 0 comma 36 crossing the x–axis at negative 6 comma 0 and 6 comma 0.

I would not normally stop to answer this question, mainly because
there is no question asked.  It's just three statements.

I do have to stop here and leave a remark, however. 

Math and Physics are closely enough related that I would not
have expected to see what I see here.  Although the math in
this question is reasonable, the Physics is inexcusable.

A rainbow is always a part of a circle.
Rainbows are never parabolas.

You could never cut a parabola out of paper, and then
hold it up in front of you after a rainstorm, and match it
to the rainbow.