That is a right triangle. That means that the other 2 angles added together equal 90, because a right angle measures 90 degrees and all the angles added together in a triangle equal 180. So the first equation is x + y = 180. If y is 15 less twice x, then 2x - 15 = y, or 2x - y = 15, if we are solving it for 15 and putting it into standard form of a line. What you are looking for is the third choice down.
