Answer
(a) $F_c=1696N$
(b) $F_N=1656N$
Work Step by Step
(a) We have the mass of the motorcycle and driver $m=342kg$, their speed $v=25m/s$ and $r=126m$
The magnitude of the centripetal force is $$F_c=m\frac{v^2}{r}=1696N$$
(b) At the top of the hill, there are 2 vertical forces acting on the motorcycle: gravitational force pointing downward and normal force pointing upward. These two forces cancel each other out to make the centripetal force found in (a). In other words,
$$mg-F_N=F_c$$ $$F_N=mg-F_c=342\times9.8-1696=1656N$$
