Where do i put the point!! Please help pre alg

What is x and y (use substitution? ) 5x+3y=38 and y=3x+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?

The area of square garden is 300m squared. how long is the diagonal?