#4 is incorrect. This statement is assuming that both 3x+5 and 4x are equal, but the correct way to use them and achieve 180 (definition of supplementary) degrees is by writing it as: 3x+5 + 4x = 180. (I spaced the 3x+5 and 4x to make it clear that these are two angles added together.) With this equation, you will be able to find the x and the substitute it back in to complete the equation.
= 6.7082