Agas powered model airplane has a mass of 2.50 kilograms. a student exerts a force on a cord to keep the airplane flying around her at a constant speed of 18.0 meters per seond in a horizontal, circular path with a radius of 25.0 meters. what is the magnitude of the centripetal force exerted on the air plane to keep it moving in this circular path

The first thing to do in this case is a free-body diagram in the radial direction.From the free body diagram you get that
 Fc = m * ar
 where
 Fc: Centripetal force
 m: mass
 ar: radial acceleration
 By definition
 ar: v ^ 2 / r
 where
 v: speed
 r: radio
 Substituting the values
 Fc = m * (v ^ 2 / r)
 Fc = (2.50 * ((18 ^ 2 / (25)) = 32.4N
 answer
 the magnitude of the centripetal force exerted on the air plane to keep it moving in this circular path is 32.4N

The answer is a.  i hope this