[m]\vec{c}=\vec{a}+\vec{b}[/m] ⇒ [m]\vec{c}\cdot \vec{c}=(\vec{a}+\vec{b})\cdot (\vec{a}+\vec{b})=(\vec{a}+\vec{b})^2[/m]
⇒ [m](\vec{a}+\vec{b})^2=(\vec{a})^2+2\vec{a}\cdot\vec{b}+(\vec{b})^2=2^2+2\cdot 2\cdot 15\cdot \frac{1}{10}+15^2=235[/m]
[m]|\vec{c}|=\sqrt{(\vec{c})^2}=\sqrt{235}[/m]