Atub of water is emptied at a rate of 3 gallons per minute. the equation y –12 = –3(x – 1) models the amount of water remaining, where x is time (in minutes) and y is the amount of water left (in gallons). analyze the work shown below to determine the initial amount of water. 1. solve for the y-variable. y – 12 = –3(x – 1) y – 12 = –3x + 3 y = –3x +15 2. write the equation using function notation. f(x) = –3x +15 the tub started with gallons of water.