Physics question about oscillation & simple harmonic motion? Help!?

Two mes are attached to each other by a string & these are attached to a vertical spring. The top m is unknown & the bottom m is 3kg. When the system is pulled downward slightly from equilibrium & released the system exhibits simple harmonic motion with a frequency of 1.8Hz. If the string holding the mes together breaks the 3kg m drops off the frequency of the system changes by a factor of 3. What is the unknown m? The spring constant "k". What is the max extension of the spring when both masses are attached? Can someone point me in the right direction & possibly give me the answers so i know i'm doing it right. Thanks.