Add my answer to Exercise 2.90.

Included is the following file:

	Exercise_2.90.scm

Thank you!
							Lawrence R. Amlord
							   Aug 2nd, 2013
							   Xi'an, China
Add my answer to Exercise 2.91.

Included is the following file:

	Exercise_2.91.scm

Thank you!
							Lawrence R. Amlord
							   Aug 2nd, 2013
							   Xi'an, China
Add my answer to Exercise 2.92.

Included is the following file:

	Exercise_2.92.scm

Thank you!
							Lawrence R. Amlord
							   Aug 3rd, 2013
							   Xi'an, China