1. |r'(t)|: \displaystyle r'(t)=\langle 3,\,4,\,0\rangle. |r'(t)|=\sqrt{9+16}=5.

2. t(s): \displaystyle s=5t\Rightarrow t=\dfrac{s}{5}

3. r(s): \displaystyle r(s)=\langle 3(\dfrac{s}{5}),\,4(\dfrac{s}{5}),\,0\rangle=\langle \dfrac{3s}{5},\,\dfrac{4s}{5},\,0\rangle.