Since the rate of change is constant this is a linear equation of the form:
y=mx+b where m is the rate of change and b is the y-intercept (the initial value in this case)
We know m=-1.5 so
y=-1.5d+b, and we know the point (8,6) so we can solve for b...
6=-1.5(8)+b
6=-12+b
b=18
So our function is:
y=-1.5x+18
Thus the initial amount of data is 18GB
