Remember that the interior angles of a triangle always add up to 180 degrees. We know that two of the angles (45 and 15) add up to 60 degrees. Now we just have to subtract the 60 degrees we have from the 180 degree total in order to get the measure of the missing angle.
Think: every point on this line features 1 as the y value. Thus, the line must be horizontal, and its equation is y = 1. The slope is 0. (First answer choice)
