First you need to solve for "x": 6x+3=8x-21, 3=2x-21, 24=2x, x=12.
Then plug in number 12 to your equation: 6(12)+3=75;
Just to make sure that I solved correctly for x, plug in 12 into the second equation: 8(12)-21=75.
Yahoo, the x is 12 and angle measure is 75°.