Answer: 
The speed of the potato at the lowest point is 6.714 m/s
The tension in the string is 8.82 N
Explanation:
We will use law of conservation of energy to find out the velocity
When the potato is at 2.30 m height, it will have potential energy and when  the potato released it will go lowest point and it's potential energy will be convert into kinetic energy.
                        Potential energy = Kinetic energy
                                   m×g×h      = (1/2) m×v²
m stands  for mass
g stands for gravitational constant
h stands for height
v stands for velocity
                       m×g×h      = (1/2) m×v²
                       0.3×9.8×2.3 = (1/2) .3×v²
                        v² = 45.08
                        v  = 6.714 m/s
The speed of the potato at lowest point is 6.714 m/s
Part B
  The potato is in uniform circular motion so the acceleration is given by
                           a = v² / r
 
a stands for centripetal acceleration
According to Newton second law the acceleration  is  directly proportional to applied force and inversely proportional to the mass of object
      So                     ∑ F = m × a
Two forces act on potato 1. weight due to gravitation in downward direction 2. tension in upward direction So the above equation become
                      Tension force - Force due to weight = m×a
                      T-m×g     = m×a 
            centripetal acceleration formula =v²/r    ,   r = l = length of the string
                         T  = m ×(v²/l) + m×g
   v = 6.714 m/s ,  l = 2.30 m ,   m = .3 kg Now put values in above equation
                          T = .3 × (6.714²/2.30) + .3 × 9.8
                         T  = .3 ( 45.08/2.30) + 2.94
                         T  = .3 (19.6) +2.94
                         T  = 5.88 + 2.94
                         T  = 8.82 N
           The tension in the string is 8.82 N