From Mechanics

First find the impulse (change in momentum).

\displaystyle \begin{align} & \text{Impulse }=mv-mu \\ & 1200\times 15-1200\times 20 \\ & =-6000\text{ Ns} \end{align}

Use the formula

\displaystyle \text{Impulse }=Ft

\displaystyle \begin{align} & -6000=F\times 20 \\ & F=-300\text{ N} \\ \end{align}