For the system shown in figure below

At point A the end of the springs are rigidly connected.

Numerical vlaues:

Spring constants: $k_1=15\frac{N}{m}$;$k_2=5\frac{N}{m}$;$k_3=30\frac{N}{m}$;$k_4=15\frac{N}{m}$ mass: $m=3kg$

a$)$ determine the equivalent elastic constant of springs $k_2$ and $k_3$