Yup, today was Cosine Law. If you have a triangle with opposite corresponding sides , you can find a missing side using
You need to know the lengths of and and the angle to do this.
You can also use the Cosine Law to find any missing angle if you know the lengths of all of the sides. In the above form, you can find angle by substituting the side lengths and rearranging.
Your homework tonight is to complete Page 409 #2 and Page 418 #2.