From D' to F' is 7.5 and from D to F is 5cm the difference is 2.5. From F' to E' is x and from F to E is 8. This is just understanding what you can see. You find x by figuring out the ratio from 5 to 7.5 is and 8 to x or vise versa. It depends which way you are going. I think you are going from E,D,F to E',D',F'. So from 5cm to 7.5 cm. This is 1.5 because you are dividing 7.5/5. If you needed to find x, it would be 12.
