-3 = 7 - BLANK pls tell me what blank is

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 is the axis of symmetry of the function f(x)=-(x+ 9)(x-21)

What makes a trapezoid an isosceles trapezoid?