Look at these adjectives:
Tom est beau, mais Henri et Bob sont très beaux.Tom is handsome, but Henri and Bob are very handsome.
To form the masculine plural, add an -x to the adjectives ending in -eau (beau/nouveau), and vieux doesn't change.
To form the feminine plural, it follows the simple rule for pluralisation, and simply takes an -s.
Case of masculine words starting with a vowel or mute h
The masculine forms bel, nouvel and vieil (used in front of a vowel or a mute h) become the regular masculine plural beaux, nouveaux andvieux.
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Examples and resources
"Plurals in French" by www.frenchspanishonline.com

Yvette est belle, mais Leila et Belinda sont très belles.Yvette is beautiful, but Leila and Belinda are very beautiful.
Son nouvel habit est plus joli que tous mes nouveaux habits.Her new outfit is prettier than all my new outfits.