Two identical springs of spring constant k are attached to a block of mass m and to fixed supports as shown in figure. When the mass is displaced from equilibrium positions by a distance x towards right, find the restoring force.

 

Hint: The two springs are in parallel combination.
Step 1: Find the net force on the block.
Consider the diagram in which the block is displaced right through x.
The right spring gets compressed by x developing a restoring force k x towards the left on the block. The left spring is stretched by an amount x developing a restoring force k x towards left on the block a given in the free body diagram of the block.
 
Hence, total force (restoring)=(k x + k x)                            [ Both forces are in same direction]
                                          = 2k x towards left