Marcus planted two trees in his backyard, an oak tree and a pecan tree. The height, in feet, of the oak tree after x years can be modeled by the expression 5x + 9. The height, in feet, of the pecan tree after x years can be modeled by the expression 3x + 15. Which graph represents the time when the height of the oak tree is greater than or equal to the height of the pecan tree?