Given the equation:
You know that the variable "g" represents the length of the average green turtle, and the variable "l" represents the length of the average leatherneck turtle.
By analyzing the equation, you can identify that the length of the average green turtle can be calculated by subtracting 3 units of length from the length of the average leatherneck turtle.
Based on the choices given in the exercise, you can identify that the unit of length for this case is "inches".
Therefore, an average green turtle is 3 inches shorter than an average leatherneck turtle.
Hence, the answer is: First option.