Answer
$8.27\times 10^{-11}m$.
Work Step by Step
The accelerating voltage gives 15 keV of kinetic energy to the electrons, which then produce the x ray photons.
$$eV=\frac{hc}{\lambda}$$
$$\lambda =\frac{hc}{ eV}=
\frac{(4.136\times10^{-15}eV \cdot s)(3.00\times10^8 m/s)}{(15000eV)}=8.27\times 10^{-11}m$$
