Explanation:
when 2 shapes are similar, it means that all their angles are the same compared with each other.
and all distances (incl. side lengths) if one shape correlate to the corresponding distances in the other shadow by the same ratio or factor.
we have one side (30 m in A and 50 m in B), where we have both measures.
we see that the ratio to "go" from A to B is 30/50 = 3/5.
that means a distance in A is the same distance in B multiplied by 3/5.
and that factor is the same also for every other side.
therefore,
x = 15 m × 3/5 = 3 m × 3 = 9 m
y × 3/5 = 18 m
3y = 90 m
y = 30 m