Answer
$9.0\times10^5\frac{m}{s}$
Work Step by Step
The kinetic energy of the particle is given. Solve for the speed. We assume this is a non-relativistic particle.
$$\frac{1}{2}mv^2=KE$$
$$v=\sqrt{\frac{2KE}{m}}=\sqrt{\frac{2(4.2\times 10^3eV)(1.60\times10^{-19}J/eV)}{1.67\times10^{-27}kg}}$$
$$=9.0\times10^5\frac{m}{s}$$
