A hall 100 feet in length is to be designed as a whispering gallery. If the foci are located 25 feet from the center, how high will the ceiing be at th center?

I believe that the minor axis, 2a is 150 feet and what Im trying to find is the major axis, 2b. So all i have to do is use this equation b2=a2-c2. b2= 75 squared - c2.

I have no idea what c is, or am i doing this completely wrong?