Answer:
 x = 259 Hz
Explanation:
given,
frequency of one tuning fork = 250 Hz
frequency of another tuning fork = 266 Hz
when a tuning fork is sounded together beat frequency heard = 9
 let x be the frequency of unknown
 x - 250 = 9 Hz..............(1)
 x = 259 Hz
when a another tuning fork is sounded together beat frequency heard = 7
266 - x  = 7 Hz..............(2)
 x = 259 Hz
now, on solving both the equation the frequency comes out to be 259 Hz.
so, The frequency of the tuning fork is equal to 259 Hz