Final answer:
A generic formula to express the length of the rectangle in terms of the number of seconds since it began growing could be l = at + b, assuming a direct linear relationship. Without specific growth data, the constant a represents the rate of change in length per second, and b would be the initial length.
Step-by-step explanation:
Based on the details provided and the reference to a rectangle 'beginning to grow,' one can assume a direct relationship between the length of the rectangle and time. Unfortunately, there is not enough specific information to create an exact formula, but a generic formula could look something like l = at + b, where a symbolizes the growth rate in inches per second, and b is the initial length at t = 0 seconds.
For the proportionality between the length and the number of seconds, a possible formula could be l = kt, where k represents a constant growth rate and t the time in seconds since the rectangle began growing. This linear relationship signifies that for every second, the length grows by a constant amount, k inches.