A proton is fired from origin with velocity $\vec{v}=v_0\vec{j}+v_0\vec{k}$ in a uniform magnetic field $\vec{B}=B_0\vec{j}$. In the subsequent motion of the proton:

A charged particle is projected in a magnetic field $\vec{B}=(3\vec{i}+4\vec{j})\times 10^{-2}T$ and the acceleration is found to be $\vec{a}=(x\vec{i}+2\vec{j})m/s^2$. The value of x is :