Answer:
Hi Sophia! The answer is the option 3!
Step-by-step explanation:
Check the picture!

Here, Cos (60°) = BC/AB = <u>a/12 units</u>
<em>Hope </em><em>this</em><em> </em><em>helps</em><em> </em><em>:</em><em>)</em>
I would help but I honestly did not understand this at all when I took Geometry last year. Also you have very pretty handwriting