Find a possible formula for $f(x)$ if $f(1) = 8 \ $ and $f(2) = 40 \ $ and:

(a) $f(x)$ is a linear function. $f(x) =$

(b) $\ f(x)$ is a power function. $f(x) =$

c) $f(x)$ is an exponential function. $f(x) =$