Chapter 4 - Sections 4.1-4.9 - Exercises - Problems by Topic - Page 189: 91c

$Ca: +2$ , $F: -1$

All Group 2A metals have a oxidation number of $+2$ so Ca has an oxidation number of $+2$. Since the sum of the oxidation states of all atoms in a neutral molecule is equal to 0. The oxidation number of F must be $-1$ so the 2 F atoms have an added oxidation number of $-2$ which added to the oxidation number of Ca would be 0.