Problems 12, 13, 14show work if any you

Evaluate the expression for the given value of the variable 7a - 4a for a =8

Mrs. morton has a special reward system for her class. when all her students behave well, she rewards them by putting 3 marbles into a marble jar. when the jar has 100 or more marbles, the students have a party. right now, the the jar has 24 marbles. how could mrs. morton reward the class in order for the students to have a party?

What are the first 4 terms in the multiplication pattern given by the formula 2×4n