Starting from a velocity of 0, your change in velocity over a period of 6 seconds will be ...
... (9.8 m/s²)×(6 s) = 58.8 m/s . . . . . downward, since you are "falling"
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Note the units of seconds in the time period cancel one of the factors of seconds in the acceleration to leave only seconds in the denominator of the result. That is confirmation that the proper math operation is being carried out, since the units of velocity are meters per second.
