when a line intersects 2 parallel line then there are some rules they follows
There are eight angles will together form four pairs of corresponding angles. Corresponding angles are congruent.
see the attached image.
in that
1 = 5
4 = 8
2 = 6
3 = 7
these are corresponding angles
also
1 = 3
2 = 4
5 = 7
6 = 8
these are opposite angles.
from above we can deduce
2 = 8
3 = 5
these called interior angles.
so in your question
angle ABC + angle CBD = 180
angle ABC + 100 = 180
angle ABC = 180-100= 80
now
Angle EFB = angle ABC
because corresponding angles.
so x = 80
