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Since bb is fairly simple, you can substitute that into sms. Then, solving gives something nice. However, you have to tell Sage some information about a.

var("x,L,a")
assume(a>0)
assume(a<1)
sms = (x/(1-L))^(1-a) == 3/10
bb = x == 3/10*L
solve(sms.subs(bb),L)