Applying a scale factor means by definition to multiply the lengths of the segments by the scale factor. If the scale factor is larger than 1, we call it a dilation.
Now, we get that AB has a length of 1.5 cm; applying the scale factor yields a new segment with 2 new endpoints that has a length of A'B'=3 cm = 1.5cm*2
Similarly, we get that since PQ=2 cm, P'Q'=2cm*2=4cm and finally since MN=3 cm, M'N'=6 cm.