51 mm. 1 centimeter = 10 millimeters. 1 millimeter = 0.1 centimeter.
http://wiki.answers.com/Q/What_is_5.1_centimeters_...

2 meters is equal to 2000 millimeters.
http://www.chacha.com/question/whatis2meterseq...

36 inches, or 3 feet.
http://wiki.answers.com/Q/What_is_1_yard_equal_to

Any nonzero fractions where the numerator and denominator are the same number, are equal to 1. For example, 2/2 is equal to 1, or 3/3, 4/4, and so on.
http://wiki.answers.com/Q/What_fractions_are_equal...
