8. suppose an investment of $5,000 doubles every 12 years. how much is the investment worth after 36 years? after 48 years? write and solve an exponential equation.

It's a doubling time question, so the formula will have the form of y = a(2)^(x/t). Y is the present amount; a is the initial amount; x is the number of year; t is the period of doubling time. 

So now you just plug in values:

y = 5000(2)^ (36/12)
y = 5000(2^3)
y = 5000 (8)
y = 40 000

And it's the same for the second part (just with 48 instead of 36):

y = 5000(2)^ (48/12)
y = 5000(2^4)
y = 5000 (16)
y = 80 000

Where are the functions?