You just need to plug those expressions inside the formula: it doesn't matter if they're expressions involving a variable instead of plain numbers: the formula becomes
If you want, you can simplify it by expanding the square and then multiply the two parenthesis:
The answer would be the Last option (D). This because it poses the proportion of 48 in (Amber's height) / 60 in (Lucy's height) and puts it equal to the proportion of x (what is Amber's height in the photo) / 3 (what is Lucy's height in the photo. Hope this helps :)
