x-1 being HCF (highest common factor), or factor of the expression implies that it divides the expression in question
So, x-1=0 is a root of the polynomial ax^2-b(x+1), this means putting 1 into the polynomial expression should give us 0
For x=1, we get a-b(1+1)=0 => a-2b=0
therefore we get, a=2b.
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