Answer
$6.63\times10^{-34}\;J.s$
Work Step by Step
$E=\frac{ch}{\lambda_{min}}$
or, $h=\frac{E\lambda_{min}}{c}$
Substituting the given values, we obtain the Planck constant:
$h=\frac{40\times 10^{3}\times1.6\times10^{-19}\times 31.1\times10^{-12}}{3\times10^{8}}\;J.s=6.63\times10^{-34}\;J.s$
