PLEASE HELP!!!! WILL GIVE BRAINLIEST!!!’

You have a rectangular space where you plan to create an obstacle course for an animal. The area of the rectangular space is represented by the expression 12x^2 - 15x. The width of the rectangular space is represented by the expression 3x
Part A: Write an expression to represent the length of the rectangular space. Then simplify your expression. Show all your work.
Part B: Prove that your answer in part A is correct by multiplying the length and the width of the rectangle. Show all your work.