In order to answer this one, it's really really really really helpful if you know what the law of cosines says. In fact it's absolutely necessary.
The law of cosines says if you know two sides of a triangle and the angle between them then you can use that information to find the length of the third side.
In the picture you know the lengths of two sides and you know the angle between them. So you can use the law of cosines to find the length of the third sidethat. That's side AC.
