For a particular scale drawing, it is already specified that all the measurements' some constant scaled version will be taken. For example, let the scale be K feet to s inches.
Then it means
All feet measurements will then be multiplied by s/k to get the drawing's corresponding lengths.
The length of AB can be written as,
AB = √[(1-4)²+ (1-1)²]
AB = 3
Since the length A'B' is 6 units, therefore, the scale factor will be,
Scale factor = (Length after transformation)/(Length before transformation)