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Initially the spring extends in length, then its restoration force on either end will be same and equal to  - k x, where k = force constant of the spring and x is the extension of the spring.  Hence block A is pulled towards right with the same spring force.

Equations of dynamics of the blocks are 
      F1 - k x  =  M_B * a_B
      k x - F2  =  M_A * a_A

    F1 - F2 = M_B * a_B + M_A * a_A 
     100 - 70  = 5 * 4 + 2 * a_A
          a_A = 5 m/sec²

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sir can you please explain where did the first equation come from?