A physics Question im stuck on

A(n) 21 kg boy rides a roller coaster.
The acceleration of gravity is 9.8 m/s
With what force does he press against the
seat when the car moving at 6.7 m/s goes over
a crest whose radius of curvature is 14 m?
Answer in units of N



2. F=ma a=(v2)/r

Fn= ma-mg



3. The attempt at a solution

Fn=ma
Fn=21kg(6.7m/s^2/14m)
Fn= 67.335 (why is this wrong?)



(my hw database says this is wrong, so i tried...)

Fn = ma + mg
Fn= 67.335 + 205.8 = 273.135

(my hw database says this is also wrong what am i missing or doing wrong?)

Gimme some help niggiz