Answer:
1. He added 5 instead of subtracted 5.
2. He should have subtracted 5.
3. 2 2/3
Step-by-step explanation:
3x+5=13
subtract 5 to both sides 3x=8
divide both sides by 3 x=8/3 or 2 2/3
Answer:
2A/b = h
Step-by-step explanation:
A=1/2bh
Multiply each side by 2
2A = 2 * 1/2 bh
2A = bh
Divide each side by b
2A/b = bh/b
2A/b = h
2.

(this is given in the first sentence)
3.

and

due to ...
7. ... the alternate interior angles theorem (which holds because

and

are parallel)
8. Because the above establishes that

, which means

, which in turn means

is a parallelogram.