The gazelle has zero average acceleration because it's moving at a constant speed. Constant speed means no change in speed means zero acceleration.
The cheetah, on the other hand, changes its speed from 0 m/min to 1820 m/min in a matter of 3 seconds, so its average acceleration is
a[ave] = (1820 m/min - 0 m/min) / (3 s)
… = (1820 m/min) / (3/60 min)
… = 36,400 m/min²
… = (36,400 m) / (60 s)²
… = 91/9 m/s² ≈ 10.1 m/s²
