Answer
$$L = \sqrt {26} $$
Work Step by Step
$$\eqalign{
& {\bf{r}}\left( t \right) = \left\langle {3t - 1,4t + 5,t} \right\rangle ,\,\,\,\,\,{\text{for }}0 \leqslant t \leqslant 1 \cr
& {\text{find }}{\bf{r}}'\left( t \right) \cr
& {\bf{r}}'\left( t \right) = \frac{d}{{dt}}\left\langle {3t - 1,4t + 5,t} \right\rangle \cr
& {\text{differentiating}} \cr
& {\bf{r}}'\left( t \right) = \left\langle {3,4,1} \right\rangle \cr
& {\text{use the Definition of Arc Length for Vector Functions }}\left( {{\text{see page 832}}} \right) \cr
& {\text{for a vector }}{\bf{r}}'\left( t \right) = \left\langle {f'\left( t \right),g'\left( t \right),h'\left( t \right)} \right\rangle \cr
& L = \int_a^b {\sqrt {f'{{\left( t \right)}^2} + g'{{\left( t \right)}^2} + h'{{\left( t \right)}^2}} dt} = \int_a^b {\left| {{\bf{r}}'\left( t \right)} \right|} dt \cr
& {\text{then}} \cr
& \,{\text{for }}0 \leqslant t \leqslant 1 \to a = 0{\text{ and }}b = 2.{\text{ then}} \cr
& L = \int_0^1 {\left| {\left\langle {3,4,1} \right\rangle } \right|} dt \cr
& L = \int_0^1 {\sqrt {{{\left( 3 \right)}^2} + {{\left( 4 \right)}^2} + {{\left( 1 \right)}^2}} } dt \cr
& {\text{simplifying}} \cr
& L = \int_0^1 {\sqrt {9 + 16 + 1} } dt \cr
& L = \int_0^1 {\sqrt {26} } dt \cr
& {\text{integrating}} \cr
& L = \left( {\sqrt {26} t} \right)_0^1 \cr
& L = \sqrt {26} \left( t \right)_0^1 \cr
& {\text{evaluating}} \cr
& L = \sqrt {26} \left( {1 - 0} \right) \cr
& L = \sqrt {26} \cr} $$