Force & velocity are perpendicular to each other in magnetic fieldSo\\\\(\\vec{V}\\cdot\\vec{a}=0\\\\)\\\\((3\\hat{i}+4\\hat{j})\\cdot(2\\hat{i}+x\\hat{j})=0\\\\)\\\\(6+4x=0\\\\)\\\\(x=-1.5\\\\)No work is done by magnetic field. So, KE of particle is constant