The net force is in the direction of the larger force. If we take that direction to be positive, then the net force has a magnitude of
68.8 N - 26.5 N = 42.3 N
Use Newton's second law to solve for the acceleration:
F = m a
42.3 N = (21.2 kg) a
a = (42.3 N) / (21.2 kg)
a ≈ 2.00 m/s²
(you might also try 1.99 or 2.01 in case that doesn't get accepted)
