In a triangle, sides a, b, and c are opposite angles A, B, and C, respectively. If a=15, B=70°, and C=80°, what's the formula for c?

You will use the Law of Sines. So first you'll need to find Angle A. To do this, you will need to add up the other angles and subtract them from 180. 70 + 80=150, and 180-150 equals 30, so Angle A equals 30. So now the formula to find c is sin 30/15=sin80/c. Cross multiply and divide by sin 30 on both sides to get c by itself. I ended up with 29.5, rounded up to 30 as my answer for c.