A ship's course is set at a heading of 143 degrees at 18 knots. A 10-knot current flows at a bearing of 112 degrees. What is the ground velocity of the ship?

Here is a rough diagram I drew:

I translated the 18 knot vector and the faded line is the resultant vector, or the ground velocity that im trying to find. In class we use the cosine law to determine the magnitude, however i'm not really sure how to find the angle opposite of the faded line.. or perhaps i'm going about it wrong?