The money in the account increases at a constant rate of $2 per week.
A function is linear if the change of the output is proportional to the change in the input. For this problem, the number of weeks is the input and the amount of money in the account is the output. As you can see in the table, for every one week, the amount of money in the account increases by $2 - this is a proportional relationship.
A linear function is one that functions without a definite value for x and y (two variables) but there is a relationship between these two variables. For example, if x is the number of weeks and y is the money in account, the linear function would be y = 2x + 4, where for every increasing y (week), the x (money in account) increases by 2 and the base amount is 4.
A nonlinear function is one that functions with a definite value for x and y (two variables) and there may be a relationship between these two variables. For example: 3x + 3y = 9 and 2x = y.
