A straight line is 180°, so you add these two angles together because they make a straight line:
3x° + 75° = 180°
To find "x", isolate/get the variable "x" by itself in the equation. Subtract 75° on both sides
3x° + 75° - 75° = 180° - 75°
3x° = 105° Divide 3 on both sides to get "x" by itself
x = 35°
PROOF
3x° + 75° = 180° Substitute/plug in 35 into "x" since x = 35°
3(35)° + 75° = 180°
105° + 75° = 180°
180° = 180°
