In triangle $ABC$, $AB = 4$ cm and angle $ABC = 120°$. The area of the triangle is $6\sqrt{3}$ cm$^2$. Work out the exact length of $AC$.

• A$2\sqrt{7}$ cm
• B$\sqrt{37}$ cm
• C$2\sqrt{13}$ cm
• D$2\sqrt{19}$ cm

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