Question 1097700
<pre>
{{{drawing(33600/89,400,-8.9,8.9,-.9,15.9,grid(1),

arc(0,0,16,-30,0,180) )}}}

That's the top half of an ellipse with the equation:

{{{x^2/b^2+y^2/a^2=1}}}

where b=8 (the semi-minor axis, or half the 16 ft. base), 
and a=15, the semi-major axis, the 15 ft arch height.

{{{x^2/8^2+y^2/15^2}}}{{{""=""}}}{{{1}}}

Simplify and solve that for y, taking only the positive
square root, and you'll get:

{{{y}}}{{{""=""}}}{{{expr(15/8)sqrt(64-x^2)}}}

Edwin</pre>