Hello, archistrategos214!
Did you make a sketch?
A tunnel has a height of 20m and a width of 36m at the base.
If the vertex is at the top of the arch, at which point above the base is it 18m wide? Code:
20
* * *
*  *
*  *
*  *

 *      +      * 
18  18
The vertex of the parabola is on the yaxis.
. . The equation has the form: .
Since is on the parabola: .
. . The equation (so far) is: .
Since is on the parabola: .
. . The parabola is: .
"Where is the parabola 18m wide?" "Where is ?"
. . When
The arch is 18m wide at a point 15m above the base.