if you look at this diagram as 2 triangles, the little one, and then the big one that the little one is inside of, and realize that they are similar triangles (by AAA standards) you can use a ration to solve the answer. the side length 9 is the length that is similar to side length 21, so you put those in a fraction format (9/21), and side length 3 is similar to side length x so you put those in a fraction (3/x), and then you create a ration with those
9/21=3/x
now you can cross-multiply 21 times three (63) and divide that number by 9 (7) to get your final answer of x=7.
