When you evaluate the function f (x) = 4 • 7 ^ x for x = -1 you get:
f (-1) = 4 * 7 ^ -1
f(-1) = 4* 1/7
f (-1) = 0.5714
The next part of the question is not clear. If it refers to the function at x = 2 then:
f (2) = 4 * 7 ^ (2)
f(2) =4*49
f (2) = 196
If it refers to it in x ^ 2
f (x ^ 2) = 4 * 7 ^ (x ^ 2)
She will have $500 in 5 days because when she takes out the 20 every week she will have had $100 so to make $500 she will have to have made 5 $100
