When the block is displaced by a distance x, force acting on block due to both springs will be in the same direction. Therefore,
F=−2kx⇒a=−m2kx,
Comparing it witha=−ω2x , we get
ω=m2k=22×20
=20rads−1
Now, T=ω2π=202π=5π
Hence, required value of X=5.