The child's radial/centripetal acceleration at the lowest point of the swing is
a = (3.00 m/s)² / (7.00 m) = 1.29 m/s²
By Newton's second law, the tension in the rope is
T = (3.5 kg) (1.29 m/s²) = 4.5 N
(Does the question really say the child's mass is 3.5 kg? That's little more than the average mass of a newborn, about 3.3 kg. Not exactly a good idea to put a newborn on a swinging rope...)
