In a geometric sequence you are just multiplying basically. To go from 36 to 24, you multiply by 2/3, and the same will be with the next term. When you multiply 24 by 2/3 you get 16.
We know that the area of a rectangle is equal to the width multiplied by the length. So in this case the algebraic expression is (x+5)*(3x+10). You get 3x^2 +10x +15x +50= 3x^2 +25x +50.
