Answer
35 kV
Work Step by Step
The shortest-wavelength x-ray has the most energy. The incoming electrons have some kinetic energy, and that is the maximum energy of an x-ray photon.
In this problem, the value of hc is useful. Use the result stated in Chapter 27, Problem 29, $hc=1240\;eV \cdot nm$.
Use equation 27–4, E = hf, and recall that $\lambda f = c$.
$$E = hf = \frac{hc}{\lambda }=\frac{1240\;eV \cdot nm }{0.035 nm}=3.5\times10^{4}eV$$
The electrons have KE of 35 keV, so the voltage across the tube is 35 kV.
