(x-a) (x-b) =0
If either (x-a)=0 or (x-b)=0 then the entire expression is equal to 0.
Set the first factor equal to 0 and solve.
Set the next factor equal to 0 and solve.
The solution is all values that make (x-a)(x-b) =0 true.
x= a, b
which in your problem is choice b) x=a, x=b