The value of x in the diagram is 4
Angles on parallel lines can be alternate , corresponding, vertical to each other. In this case they are equal to each other.They can be supplementary too.
Supplementary angles are angles that add up to 180°. The angles are mostly adjascent to each other.
In the diagram above ;
21x + 6 = 90 ( alternate exterior angles)
21x = 90 -6
21x = 84
divide both sides by 21
x = 81/24
x = 4
Therefore the value of x in the diagram is 4