The given answer "fifty-first" is the correct way to write the ordinal number for 51st. In English, ordinal numbers are formed by adding the suffix "-th" to the cardinal number, except for numbers ending in 1, 2, or 3, which use the suffixes "-st", "-nd", and "-rd" respectively. Since 51 ends in 1, it follows the pattern and the correct ordinal form is "fifty-first".