We can find x only if we know that the figure is a REGULAR octagon, i.e. an octagon with all sides equal, and all interior angles equal.
We will ASSUME that the figure is a regular octagon.
For a regular polygon, the central angle subtended by each side is equal.
The central angle is 360 degrees.
So the central angle subtended by a single side euqals
a=360 degrees / 8 sides = 45 degrees / side
Since x is the total angle subtended by three sides, so
x=3*45=135 °
