Here we can see that written work done = (+) delta change in P.E But we know that work done by conservative force is - (-) delta change in potential energy , is it verifying magnetic force is non conservative or here something like that Wext + Wmagnetic = delta K.E = 0 is using so W ext = - W magnetic .
W = Change in potential energy = U2-U1
U2 = -(MB)Cos(60)
U1 = -(MB)Cos(0)
Plz, check & read your asked question again. You are misunderstanding.