Question

a-1) first you stretch the spring by moving the block to a positive displacement, x = a. what is the force of the spring on the block pulling back? (indicate magnitude and direction.)

Answer 1

The force's magnitude will be: F = k·|a| with k being the spring's elastic constant and |a| is the absolute value of the displacement.

The direction can be any, as depends on the reference system you choose. If the reference system is directed in the opposite way of the elastic force, it will be negative, else positive, but in any case the elastic force will be directed towards the equilibrium position of the spring.