Question 1062337
Where is your origin?  How high is the base of the bridge above the water?


With just the given, you might put the bridge base at the water level and assign the origin at the middle between the two sides of the water (stream or creek or river).  This way, your zeros are at (-25,0) and (25,0), and vertex as maximum height is (0, 4.5).  Your unknown constant factor in  {{{y=a(x-0)^2+4.5}}} can be determined.