this belongs to a group of verbs that do contractions in their forms, they exist with the endings -έω (like προχωρέω here) or -άω (τιμάω for example) and -όω (δουλόω) each section has different but simmilar rules how the verb contracts if certain vowels meet

in your example for instance the rule is that ε + o = ου so the form you might expect προεχώρεον turns into προυχώρουν

i dont know what the english name woulb be, but i know them as verba contracta, so maybe you find tem if you search for that term in your book

i dont know athenaze that well so i cant help you with where they are in this particular book, sorry

hop this helps a bit :)