Although it is unclear as to what the exponent(yen symbol) of 8.85 x 10^¥ is, since it has been stated that it is an FM wave, let us assume that the exponent is 6. An FM wave has frequencies from 535-1605 kHz, raising the given frequency to 6 allows it to fit into that range. Given the frequency, the period can be obtained by taking the reciprocal of the frequency. This is done below.
Period = 1 / frequency = 1 / 8.85 x 10^6
Period = 0.0000001129943503 = 112.99 nanoseconds