To find the scale factor, we divide one side by another that we know is correlated to that side (for example, if the sides are parallel). Since we can see that SR lines up with S'R' and RQ lines up with R'Q' (since it's in the same pyramid, they're parallel), they are similar. Finding the ratio, we get 6/15 or 4/10 or 2/5. That means that S'P' * 2/5=SP etc..