(a) My workings:

Mg-T=M*((Mg)/(M+m))

T=Mg(1-M/(M+m))

T=$\displaystyle \lambda$x/l

x=(Mgl/ $\displaystyle \lambda$)(1-M/(M+m))

Mark Scheme:

I understood the mark scheme, but I don't understand what went wrong with my working, ie where is my logic faulty and what effect i have ignored. I would appreciate it if you can explain my mistake