similar triangles share the same 3 angles, and the ratio of each set of corresponding sides is the same in all 3 cases.
The shortest side of the first triangle has length 9, whereas the length of the corresponding side of the 2nd triangle has length 7.
Think: "the ratio mentioned above is (9/7), or 9:7.
Set up an equation of ratios to find the unknown length of the middle side of the smaller triangle:
9 13
-- = ------
7 x
Solve this for x. Perform a similar operation to find the longest side of the 2nd (smaller) triangle.