What is the Latin stem of the word "mae" adj. meaning my?

What is the lating stem of the word “mae” an adj. meaning my? in class i had an exam and had to decline a noun/adj pair with the pair being “mea poena” as i was taught in class i took the “ae” off then end of mea making my stem “m”. with this i proceeded to fill in the table of nominative, genitive, dating, accusative, ablative, and vocative singular and plural forms of both
ex. Gen sing= mae poenae. dat sing= mae poenae. dat plur= mis poenis. all of my declinations fo the poenae word was fine but the took off 3 points for my stem of mea. Can anybody tell me what it is so i can correct this and return it to her please????