Hello, magentarita!
The cables of a suspension bridge are in the shape of a parabola.
The towers supporting the cable are 400 feet apart and 100 feet high.
If the cable is at a height of 10 feet midway between the towers,
what is the height of the cable at a point 50 feet from the center of the bridge? Place the parabola on a graph. Code:

(200,100)*  *(200,100)
:  :
:*  *:
: *  * :
: *  * :
: * :
: (0,10) :
:  :
  +    +    + 
200  200
The parabola opens upward and is symmetric to the yaxis.
. . Its general form is: .$\displaystyle y \;=\;ax^2 + c$
Its yintercept is (0, 10) . . . Hence: .$\displaystyle y \;=\;ax^2 + 10$
It passes through (200, 100).
. . We have: .$\displaystyle 100 \:=\:a\cdot200^2 + 10 \quad\Rightarrow\quad a \:=\:\frac{9}{4000}$
Hence: .$\displaystyle y \;=\;\tfrac{9}{4000}x^2 + 10$
When $\displaystyle x = \pm50,\;\;y \:=\:\tfrac{9}{4000}(50^2) + 10 \:=\:\frac{125}{8}$
Therefore, 50 feet from the center, the cable is $\displaystyle 15\tfrac{5}{8}$ feet high.