Motion in a Straight Line

A particle is moving in one dimension (along x axis) under the action of a variable force. It's initial position was 16m right of origin. The variation of its position (x) with time (t) is given as x=−3t3+18t2+16t, where x is in m and t is in s. The velocity of the particle when its acceleration becomes zero is_______ m∕ s.