HP 39g+ - Manual (Page 115)

Statistics aplet 8-21
You can type PREDX and PREDY into the edit line, or you
can copy these function names from the MATH menu
under the Stat-Two category.
H I N T In cases where more than one fit curve is displayed, the
PREDY function uses the most recently calculated curve. In
order to avoid errors with this function, uncheck all fits
except the one that you want to work with, or use the Plot
View method.