Two springs are attached in opposite directions of the block. The springs are attached to the block in parallel. The fact that they are in opposite directions does not make a difference as the resistive force created when a spring is stretched or compressed is the same, only in the direction is reversed in either case.
The equivalent resistance of the springs is Ke where 1/Ke = 1/K1 + 1/K2. Here, the spring constant of the springs is 8 and 24. Ke = (8*24)/(8 + 24) = 8*24/32 = 6 N/m
The frequency of oscillation of a harmonic oscillator created with a spring of constant k and mass m is f = (1/2*pi)*sqrt(k/m)
This gives the required frequency of oscillation as f = (1/2*pi)*sqrt 6
The frequency of oscillation of the block attached to the springs is sqrt 6/2*pi Hz.