The correct answer is (x-3).
An easy way to figure this out is to find two numbers that multiply to -21 and add to 1 (however, make sure you keep in mind the leading coefficient of 2).
The easiest way is to find factors that multiply to the constant term, and then check and see whether the linear variable terms (when multiplied out) add up to the term we see in the quadratic.
Basically, just use a guess and check method by multiplying out your educated “guess” factors.
Hope this helps!