Velocity, measured in meters per second, is the rate of change of position (it tells us how much the position of the ball changes in a given time). Determining the derivative of a function is the same as determining the rate of change of the function.
So to get the velocity after 3 seconds, first get the derivative s ' (t).
s(t) = 4.9t^2+550
Use the product rule to derive:
s'(t) = (2)(4.9)t + 0
This simplifies to s'(t) = 9.8t
Since you're solving for the velocity after 3 seconds, plug in t=3:
s'(t) = 9.8 * 3 = 29.4 m/s