A square pyramid is shown with the following dimensions, splitted by a middle base. Length of middle base is 8, height of upper square pyramid is 2x+3, height of whole square pyramid is 6x+9.

Find an expression for the length L of the base of the pyramid in terms of the variable, x.
Please directly answer the question and provide the final value, e.g., 1, 2.5, 300.
Question: A square pyramid is shown with the following dimensions, splitted by a middle base. Length of middle base is 8, height of upper square pyramid is 2x+3, height of whole square pyramid is 6x+9.

Find an expression for the length L of the base of the pyramid in terms of the variable, x.
Please first conduct reasoning, and then answer the question and provide the final value, e.g., 1, 2.5, 300, at the end.
Question: A square pyramid is shown with the following dimensions, splitted by a middle base. Length of middle base is 8, height of upper square pyramid is 2x+3, height of whole square pyramid is 6x+9.

Find an expression for the length L of the base of the pyramid in terms of the variable, x.