so hmm notice the picture below
based on the info, we can say, is a parabola with that equation
notice, the towers are 40 inches apart, -20 to 20
hmmm ok, so, what's the coefficient "a"?
well, we also know the parabola has a point at 20,4 and -20, 4
so, let's use 20,4
[tex]\bf y=ax^2\qquad
\begin{cases}
x=20\\
y=4
\end{cases}\implies 4=a20^2\implies \cfrac{4}{400}=a\implies \cfrac{1}{100}=a
\\\\\\
thus\implies y=\cfrac{1}{100}x^2\iff y=\cfrac{x^2}{100}[/tex]
now... notice in the picture, the red point at (x, 0.64)
so, what's "x" when y = 0.64?
well [tex]\bf 0.64=\cfrac{x^2}{100}\implies 64=x^2\implies \pm\sqrt{64}=x\implies \pm 8 = x[/tex]
so.. now we know "x" is 8 or -8, thus the points are 8, 0.64 on the 1st quadrant and -8, 0.64 on the 2nd quadrant
so.. .let's say, let us use the 8, 0.64 point
[tex]\bf \textit{distance between 2 points}\\ \quad \\
\begin{array}{lllll}
&x_1&y_1&x_2&y_2\\
% (a,b)
&({{ 8}}\quad ,&{{ 0.64}})\quad
% (c,d)
&({{ 20}}\quad ,&{{ 4}})
\end{array}\qquad
% distance value
d = \sqrt{({{ x_2}}-{{ x_1}})^2 + ({{ y_2}}-{{ y_1}})^2}[/tex]
and surely you'd know what that is
