Solving for frequency deviation, $\Delta f $ (FM modulation)
If you use the standard definition of frequency deviation as the


  "maximum instantaneous difference between an FM modulated frequency and the nominal carrier frequency"


then for the given example you indeed have $\Delta f=50\;\text{kHz}$, because the nominal carrier frequency is $f_c=10\;\text{MHz}$, the maximum frequency is $f_c+\Delta f=10.05\;\text{MHz}$, and the minimum frequency is $f_c-\Delta f=9.95\;\text{MHz}$.