part A)
by linear momentum conservation
m1v1 =m2v2
1.3 v1 =2.85*1.2
v1 =2.63 m/s......speed of block A
Part B)
potential energy was stored in the compressed spring =1/2
m1v12 +1/2 m2v22
                                                                          Â
=0.5*1.3*(2.63)2 +0.5*2.85*(1.2)2
                                                                                                   Â
=6.5J