3 A particle \(P\) is projected vertically upwards from a point \(O\). When the particle is at a height of 0.5 m , its speed is \(6 \mathrm {~m} \mathrm {~s} ^ { - 1 }\). Find
- the greatest height reached by the particle above \(O\),
- the time after projection at which the particle returns to \(O\).