Answer
$60 ^\circ$
Work Step by Step
$a \cdot b= |a| |b|cos \theta$
$ |a| |b|=\frac{\sqrt 3}{cos \theta}$
$tan \theta=\frac{1}{\sqrt 3}| \lt 1,2,2 \gt|=\frac{\sqrt {1+4+4}}{\sqrt 3}$
$=\frac{3}{\sqrt 3}$
$arctan\frac{3}{\sqrt 3}=60 ^\circ$