Explanation

(a) The cut-off wavelength $\lambda_{\min}\ $ corresponds to an electron transferring (approximately) all of its energy to an X-ray photon, thus producing a photon with the greatest possible frequency and least possible wavelengh.
From relation
${\lambda\ }_{\min} = \frac{hc}{K_{o}}$
=$\frac{\left( 4.14 \times 10^{- 15} \right)\left( 3 \times 10^{8} \right)}{35.0 \times 10^{3}}$
= 3.55$\times 10^{- 11}\ m = 35.5pm$