Hello!
I just completed this test and if I'm correct in assuming that this is the same problem I did, then your answer would be 18 cm.
Hope this helps! :)
Fraction adding: Okay, so if you have two fractions with the same denominator like 2/3 and 1/3 then they are easily added. Just add the numerators together and then keep the denominator. Like this: 2/3 + 1/3 = 3/3 or 1 whole. Here is something for fractions with unlike denominators.