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Col A: AB
Col B: BC
If we drop a perpendicular from B to the midpoint of AC which I will call D we can treat the left triangle as a right-angled triangle with hypotenuse AB
$\displaystyle cos(30) = \frac{\sqrt{3}}{2} = \frac{Adjacent}{Hypotenuse}$
The adjacent is $\displaystyle AD = \frac{1}{2}AC = 3$
Once you know AC you can find out BC using the cos rule