The spring constant of the parallel combination of springs of spring constants k1 and k2 is kp=k1+k2=k+k=2k
This combination is in series with the spring of spring constant k3. So the effective spring constant of the system is. keff1=kp1+k31=2k1+2k1=k1 ∴keff=k
and T=2πkeffm=2πkm