If you examine the triangles and the length DE and AB you'll notice they're similar, by a scale factor of 2, so you can get your value of AC by doubling your value of CE, which is 12cm.
A highest common factor means that the number stated in the question (15) is the greatest possible number that goes into the ones you are looking at. For example, the greatest common factor of 12 and 16 is 4, because any number higher than 4 will not fit into both numbers.