13. Given that the angle between $\boldsymbol{a}$ and $\boldsymbol{b}$ is $120^{\circ}$, and $|\boldsymbol{a}|=2,|\boldsymbol{b}|=5$, then $(2 \boldsymbol{a}-\boldsymbol{b}) \cdot \boldsymbol{a}=$
$$
\boldsymbol{a} \cdot \boldsymbol{b}=-5 \Rightarrow (2 \boldsymbol{a}-\boldsymbol{b}) \cdot \boldsymbol{a}=2 \boldsymbol{a}^2-\boldsymbol{a} \cdot \boldsymbol{b}=8+5=13 .
$$