Ive a maths question based on a circle. Please help.?

The questions is to find the radius of a wonder building. It is basically just an arc. It says the span is 30 meters and the height is 10. I drew a line along the base of the arc and made that my x axis and drew a line up the centre finding my y axis. I found the centre by placing two random chords and bisecting them and where the lines meet it's my centre. I'm trying to find the radius. The answer is 16.25m but I dont know how to find it. Please help, I'm Usually good at this type of stuff and I'm sad because I can't figure it out. I bet there is something real simple that I'm just forgetting. Any who thank you!