Let's convert first the wavelength into meters:
[tex]\lambda = 0.15 nm = 0.15 \cdot 10^{-9}m[/tex]
Then, we know that the wavelength is related to the frequency by the following equation:
[tex]f = \frac{c}{\lambda} [/tex]
Where [tex]c=3 \cdot 10^8 m/s[/tex] is the speed of light in free space. Therefore, putting in the numbers, we find the frequency f:
[tex]f= \frac{3 \cdot 10^8 m/s}{0.15 \cdot 10^{-9}m}=2 \cdot 10^{18}Hz [/tex]
