The total number of subsets of {a, b, c} is 6 7 8

Do you mind if you guys me with this question i’m on number 8

An object in geometry with no width, length or height is a(n):

Carli has 90 apps on her phone. braylen has half the amount of apps as theiss. if carli has three timesthe amount of apps as theiss, how many apps does braylen have? let b represent the number of braylen's apps and t represent the number of theiss's apps.