Answer
$$\left. a \right)L = \int_0^4 {\sqrt {17 - 16x + 4{x^2}} } dx,\,\,\,\,\left. b \right)\,\,L \approx 9.29$$
Work Step by Step
$$\eqalign{
& y = 4x - {x^2}{\text{ on the interval }}\left[ {0,4} \right] \cr
& {\text{Definition of Arc Length for }}y = f\left( x \right): \cr
& {\text{Let }}f{\text{ have a continuous first derivative on the interval }}\left[ {a,b} \right]{\text{ }} \cr
& {\text{The length of the curve }} \cr
& {\text{from }}\left( {a,f\left( a \right)} \right){\text{ to }}\left( {b,f\left( b \right)} \right){\text{ is }}L = \int_a^b {\sqrt {1 + f'{{\left( x \right)}^2}} } dx \cr
& {\text{Notice that }}y = f\left( x \right) = 4x - {x^2}{\text{ and }} \cr
& \left[ {0,4} \right] \to a = 0{\text{ and }}b = 4.{\text{ Then}} \cr
& \cr
& \left. a \right)\,\,f'\left( x \right) = \frac{d}{{dx}}\left[ {4x - {x^2}} \right] \cr
& f'\left( x \right) = 4 - 2x \cr
& {\text{Using the arc length formula}}{\text{, we have}} \cr
& L = \int_0^4 {\sqrt {1 + {{\left( {4 - 2x} \right)}^2}} } dx \cr
& L = \int_0^4 {\sqrt {1 + 16 - 16x + 4{x^2}} } dx \cr
& L = \int_0^4 {\sqrt {17 - 16x + 4{x^2}} } dx \cr
& \cr
& \left. b \right){\text{ Use technology to evaluate or approximate the integral}} \cr
& L \approx 1.08 \cr} $$