Draw a picture, let the equation of arch circle be y = ax 2+bx+c, and the vertex is on the y axis.

If the points on both sides of the span are on the X axis, the equation passes through points (-18.75,0), (18.75,0), (0,7.2), and these three points are substituted into the equation to solve a, b, c, b, c.

If the vertex of the arch circle is on the X axis, the equation passes through points (-18.75, -7.2), (18.75, -7.2), (0,0), and these three points are substituted into the equation to solve A, B, C, B, C.