As a line approaches a horizontal asymptote, the y value of the graph approaches the y value of the horizontal line. However, the y value of the line will never reach the y value of the horizontal line, it merely approaches as close as possible to it.
(y-2)4 + 21 You subtract two because the 21 accounts for it and then whatever you have left multiplied by 4 and added to 21 is the amount of years after conversion.
