First of all you should demand your teachers stop teaching you algebra in geometry class.
1
Linear pairs (and this is the geometry part) are angles that add to 180 degrees, supplementary angles formed by two lines crossing.
3n + 21 + 2n + 34 = 180
5n + 55 = 180
5n = 125
n = 25
mEFG = 3n + 21 = 96 degrees
mCFH = 2n + 34 = 84 degrees
These are indeed supplements, adding to 180 degrees.
2
ACD is a right angle because it makes a linear pair along DF with another right angle.
So
mACB + mBCD = 90 degrees
mBCD = 90 - mACB = 90 - 68
mBCD = 22 degrees
