Explanation:
similar triangles (or any similar shapes) have the same angles, and there is one vegan scaling factor applying to all corresponding sides or heights or any other lines of the triangles.
so, we need to find the scaling factor from a pair of sides that we fully know.
we see that the longest side in the larger triangle is LK (= 30). and the longest side of the smaller triangle is CB (= 10). since we know that the triangles are similar, these 2 sides must correlate to each other.
LK = CB × f
30 = 10 × f
f = 30/10 = 3
JK = AB × f
21 = 7 × 3 = 21
correct
so,
JL = AC × f
JL = 5 × 3 = 15