Because these triangles are similar, their sides exist in proportion to one another. Their angles are exactly the same. but their sides are proprtionate IF they are similar. We are told they are so setting up the proportion:
and cross multiply:
4s = 12 so
s = 3
You could also look at the fact that the height of the larger triangle is 4 and the height of the smaller is 2, so the larger is twice as big as the smaller; likewise, the smaller is half the size of the larger (that means the same thing). So if the larger side is 6, half of that is 3.
This question is asking you for the least common multiple of 60 (seconds in 1 minute) and 24 (seconds in 24 seconds). I think it's 120 (you should check it) and that would be 3 minutes.