I have to use the work-energy theorem to find the coefficient of kinetic friction.

What I've done so far is:

total work done on the 6 kg block must be -0.5*6*0.9*0.9 = 2.43 J

So the net force must be -2.43J/2m = -1,215N

Fy=-mg+T=-1,215 so the tension is 57.58 N

Total work done on the 8 kg block must be -0.5*8*0.9*0.9 = -3.24 J

So the net force on the block must be

Fx=57.58-fk=-3.24J/2m

coefficient of friction must be 55.96/8*9.8=0.714

I don't get the right answer which is 0.786

Could someone please give me a hint to what I am doing wrong.

With thanks in advance