All the angles have to equal 360 degrees. so that means you would add all the angles together. Since there is one variable and not a lot of multiplication or other things you can just add all like terms so you would have a constant and then an x term that would sum up to 360, make sure to take into account the signs of each number. then solve for x by adding 15 to both sides and then dividing by 25 on both sides to get x=15.
