There are two quadratic functions. The red one is g(x) that goes through the origin and point (2, -4). Find the equation of the dashed line in red. Use exact numbers.

Please directly answer the question and provide the final value, e.g., 1, 2.5, 300.
Question: There are two quadratic functions. The red one is g(x) that goes through the origin and point (2, -4). Find the equation of the dashed line in red. Use exact numbers.

Please first conduct reasoning, and then answer the question and provide the final value, e.g., 1, 2.5, 300, at the end.
Question: There are two quadratic functions. The red one is g(x) that goes through the origin and point (2, -4). Find the equation of the dashed line in red. Use exact numbers.
