Answer:
because internal staggal angles are equal
Step-by-step explanation:
The first reason is wrong.
Angle EGH and DEG are internal staggal angles:
the two angles are on both sides of the cut line EG, and the two angles are between the two divided lines.
<em>{the definition of internal staggal angle}</em>
sweet
The answer is 21 if you use PEMDAS.