Answer
$$L = 4\sqrt 5 $$
Work Step by Step
$$\eqalign{
& y = 2x + 4{\text{ on }}\left[ { - 2,2} \right] \cr
& {\text{Definition of Arc Length for }}y = f\left( x \right): \cr
& {\text{Let }}f{\text{ have a continuous first derivative on the interval }}\left[ {a,b} \right]{\text{ }} \cr
& {\text{The length of the curve from }}\left( {a,f\left( a \right)} \right){\text{ to }}\left( {b,f\left( b \right)} \right){\text{ is }}L = \int_a^b {\sqrt {1 + f'{{\left( x \right)}^2}} } dx \cr
& {\text{Notice that }}y = f\left( x \right) = 2x + 4{\text{ and }}\left[ { - 2,2} \right] \to a = - 2{\text{ and }}b = 2.{\text{ then}} \cr
& f'\left( x \right) = \frac{d}{{dx}}\left[ {2x + 4} \right] \cr
& f'\left( x \right) = 2 \cr
& {\text{Using the arc length formula}}{\text{, we have}} \cr
& L = \int_{ - 2}^2 {\sqrt {1 + {{\left( 2 \right)}^2}} } dx \cr
& {\text{simplifying}} \cr
& L = \int_{ - 2}^2 {\sqrt 5 } dx \cr
& {\text{integrate}} \cr
& L = \sqrt 5 \left( x \right)_{ - 2}^2 \cr
& L = \sqrt 5 \left( {2 - \left( { - 2} \right)} \right) \cr
& L = 4\sqrt 5 \cr} $$