Math test , due inna couple hours .

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

Answer asap and if you do you will get brainliest. catherine buys a gallon of ice cream from the store. after taking it home, she eats a fifth of a gallon of ice cream. her sister eats some of the ice cream as well. if two-thirds of the original amount of ice cream is left, then what fraction of a gallon of ice cream did her sister eat?

Which graph shows the solution set of x^2+4-12x/> 0