Can someone explain this to me

The probability that an adult possesses a credit card is .70. a researcher selects two adults at random. by assuming the independence, the probability that the first adult possesses a credit card and the second adult does not possess a credit card is:

Given the two similar triangles, how do i find the missing length? if a=4, b=5, and b'=7.5 find a'

Having a hard time understanding this question (see attachment).