The points \(A\) and \(B\) are at the same vertical height \(h\text{ m}\) above horizontal ground. A particle \(P\) is released from rest from \(A\). One second later, a particle \(Q\) is projected vertically downwards from \(B\) with speed \(18\text{ m s}^{-1}\).

Given that \(P\) and \(Q\) reach the ground at the same time, find the value of \(h\).