Upper two springs are in series.
Therefore, their resultant spring constant k1, =k1+k1=k2 ∴k′=2k Lower two springs are in parallel.
Therefore their resultant spring constant, k′′=k+k=2k
Now, springs of constant k′ and k′′ are in series.
Therefore, their resultant spring constant k11=k2+2k1=2k4+1 k1=52k ∴ Frequency of oscillation f=T1=2π1Mk1 =2π15M2k