Answer
$135^{\circ}$
Work Step by Step
To obtain the angle $\theta$ between two non-zero vectors $a$ and $b$, we will use the following formula such as:
$ \theta =\cos^{-1} (\dfrac{\overrightarrow{a} \cdot \overrightarrow{ b} }{|a| |b|})$
We have: $\overrightarrow{a} \cdot \overrightarrow{ b}= (2)(-3) +(1) (1) =-5$
and $|a|=\sqrt {2^1+1^1}=\sqrt 5; |b|=\sqrt {(-3)^1+1^1}=\sqrt {10}$
Therefore, $ \theta =\cos^{-1} (\dfrac{-5 }{\sqrt 5 \sqrt {10}})=135^{\circ}$