An investment of $5000 doubles in value every decade. the function f(x) = 5000 · 2^x, where x is the number of decades, models the growth of the value of their investment. how much is the investment worth after 30 yr?

1 decade = 10 years
30 years = 3 decades

Simply plug in the value of the number of decades needed to find into where the x value is. 
f(x) = 5000 · 2^x
f(3) = 5000 · 2^3 = 40000
The investment is worth $40000 after 30 years.

A= r2 × 3.14 11×11 ↓ ↓ 121 × 3.14 = 379.94 → 379.9