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as i can see you are off line i changed to q&a mode
the best way to strengthen your legs in your case no doubt is swimming. this takes out the variables of the leg length discrepancy and any gait issues that may cause other problems with traditional weight training. try using a kick board to focus on the legs and not the arms and shoulders. you can progress to water running which is a hard workout but weightless running with a weight belt to hold you in place.
when you do traditional leg extensions on machines with weight do one leg at a time since when doing them together the weaker is going to compensate and you wont get equal benefit.