The following reaction is known as: \[ \text{C}_6 \text{H}_5 \text{COOH} + \text{CO}_2 \xrightarrow{120-140^\circ C, 1.5 \, \text{atm}} \text{C}_6 \text{H}_4 \text{COONa} \xrightarrow{\text{NaOH}} \text{C}_6 \text{H}_5 \text{COOH} \]
A particle is moving in a straight line. The variation of position $ x $ as a function of time $ t $ is given as: $ x = t^3 - 6t^2 + 20t + 15 $. The velocity of the body when its acceleration becomes zero is: