From the top of a tower, a ball is thrown vertically upward which reaches the ground in \( 6 \, \text{s} \). A second ball thrown vertically downward from the same position with the same speed reaches the ground in \( 1.5 \, \text{s} \). A third ball released, from the rest from the same location, will reach the ground in _________ \( \text{s} \).
