It's important to know that the slope of a linear equation refers to the constant rate of change of the variables. In this case, we have time as variable x and value (dollars) as variable y.
So, the slope of 30 means a rate of value of $30 per year.
On the other hand, the y-intercept (720) refers to the initial condition, that is, the initial value of the collection.
So, the y-intercept means the initial value of the collection of $720.
Take into account that the slope always represents a constant rate of change, while the y-intercept represents the initial condition of the problem.