Rob is saving money for a down payment on a house. he opens a savings account at his local bank and deposits $1000. he models his savings plan with the equation y = 400x + 1000 based on his current income and monthly savings rate. what is the meaning of the y-intercept in the equation? a) largest amount that he can save b) date when he will have enough saved c) date his savings account was started d) starting amount in his savings account