Where the graphs of the functions intersect it represents the number of weeks when they have both saved the same amount of money. So by solving the system we can find the number of weeks and that amount of money.
y = x^2 + 2x
y = -3x + 6 so
x^2 + 2x = -3x + 6
x^2 + 5x - 6 = 0
(x + 6)(x - 1) = 0
So x = 1 ( we ignore the negative)
So after 1 week they have saved the same amount and the amount is -3 + 6 = 3.
Are you sure that -3x + 6 is right - because Sarah does not save anything after year 1 . The equation has a negative slope!!
