fix typo in solution to Exercise 6.4-1.

Included are the following 2 files:

	6.4-1.pdf
	6.4-1.tm

Thank you!
							Lawrence X. Amlord
							  Jun 20th, 2014
							  Xi'an, China
add solution to Exercise 6.4-2.

Included are the following 2 files:

	6.4-2.pdf
	6.4-2.tm

Thank you!
							Lawrence X. Amlord
							  Jun 20th, 2014
							  Xi'an, China