Answer :
i would say 18 because if they are similar they would need to have the same starting point so if the first was multiplied than so should the second.
You can set up fractions so you can do width as the numerator or the denominator, it doesn't matter. So 4/6 = 12/x, then you can cross multiply and divide so 6(12)= 72 then divided by 4 is 18 so the length of the similar triangle is 18 inches.