A scale is just a fancy maths word for Ratio or Portion.
A scale factor can be defined when comparing two objects, by its ratio. A good example to think about, would be a Penny-Fart wheel. One wheel is larger than the other, right? Now think about the ratio which the larger is bigger than the smaller one. What would the ratio be? The scale factor, which the small one increased by, to get to the larger one? Lets say one was 4 cm and the other was 8 cm. The ratio would be 4:8.
Also, key points would be to think about the Formula. Like every maths problem, the formula is always an important aspect and plays a huge role in solving the problem.
Also, another key thing to think about are the two IMPORTANT WORDS, used in this. RATIO(which I already mentioned earlier) and SIMILAR FIGURES - The corresponding angles are equal, also their corresponding sides are proportional.
I hope this helped you :0