We know that
• The length of the pool is 20 meters.
,
• The time spent to swim the pool is 4 seconds.
,
• The motion is constant.
Given that there's a constant motion, we can use the equation d = v*t, where d = 20m, and t = 4sec. Let's find the constant velocity.

The constant velocity is 5 m/s.
On the other hand, the equation of motion would be d = v*t because the motion has a null acceleration.
The equation of motion is d = v*t.