The capacitive reactance of a capacitor is X = 1 / 2·π·f·C .
f = the frequency of the voltage across the capacitor
C = the capacitance of the capacitor.
When the capacitive reactance is 200 ohms, 200 = 1 / 2·π·f·C .
Multiply each side by (2·π·f·C): 200 · 2·π·f·C = 1
Divide each side by (400·π·C): f = 1 / 400·π·C .
f = the frequency of the voltage across the capacitor
C = the capacitance of the capacitor.
So the frequency is 1 / 400·π·(the capacitance of the capacitor).
(It makes a big difference what the value of the capcitor is. It makes such a big difference that I'll just bet the value of the capacitor is printed right there where you got the question from, and you neglected to copy it.)