The rule of 72 says to divide 72 over the interest rate without the percent sign attached to it. This means we divide 72 over 9.6 to get 72/9.6 = 7.5
So the money doubles every 7.5 years. When another 7.5 years rolls around, a total of 2*7.5 = 15 years has gone by. At this point the money is roughly 4 times that of the original amount deposited.
In the equation of a straight line (when the equation is written as "y = mx + b"), the slope is the number "m" that is multiplied on the x, and "b" is the y-intercept (that is, the point where the line crosses the vertical y-axis)
