If the line were to continue down to form a triangle at the bottom, it would cross the line with an interior angle of 70°. Since we have two of the angles, we subtract them from 180°.
180-70-50=60°
That would be the third interior angle at the top of the triangle. Angle x is supplementary to that.
We can divide the 300 by 900 to get the decimal equivalent of the percentage 300/900 = 1/3 = .33333 multiply that by 100 gives you a percentage of 33.33% increase in mark-up
