Velocity field is $\vec{V}=3y\hat{i}+4x\hat{j}$
Here, $\vec{V}=u\hat{i}+v\hat{j}$
Acceleration of the fluid particle in the $x$-direction is given as $$a_{x}=u\frac{\partial u}{\partial x}+v\frac{\partial u }{\partial y}$$ $$\Rightarrow a_{x}=3y\frac{\partial }{\partial x}\left ( 3y \right )+4x\frac{\partial }{\partial y}\left ( 3y \right )$$ $$=\left ( 3y\times 0 \right )+\left ( 4x\times 3 \right )=12x$$ At point $(x,y)$=$(3,1)$, acceleration of the fluid particle $=12x= 12\times 3= 36\,m/s^{2}$