Answer:
v = 17.30 m / s
Explanation:
For this exercise we will use Newton's second law
at the bottom of the loop and stopped
      ∑ F = 0
      N-W = 0
      N = W
      W = 770 N
the mass of the body is
      W = mg
       m = W / g
      m = 770 / 9.8
      m = 78.6 kg
on top of the loop and moving
      ∑ F = m a
      N + W = m a
note that the three vectors go in the same vertical direction down
     Â
the centripetal acceleration is
      a = v² / r
we substitute
      N + W = m v² / r
      v = [tex]\sqrt{(N+W) \frac{r}{m} }[/tex]
let's calculate
     v = [tex]\sqrt{ (350+770) \frac{21}{78.6} }[/tex]
     v = 17.30 m / s