The acceleration of gravity is 9.8 m/s. So in 2 s of falling, an object gains "(9.8\\times 2) = 19.6 \\;m\/s" of downward speed.
The balloon was already falling at 5 m/s when the time began, and it gained another 19.6 m/s on the way. So at the end of 2 seconds, its downward speed was "(5.0 + 19.6) = 24.6 \\;m\/s" .
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