So a straight line is called a supplementary angle, which means that it is equal to 180. The boxy thing on the left-hand side indicates that that angle is equal to 90 degrees.
The equation would be : 3x - 9 + x + 7 +90 = 180
Combine like terms : 4x + 88 = 180
Subtract 88 from both sides : 4x = 92
Divide by 4 on both sides : x = 23