Chemistry: The Central Science (13th Edition)

$MoO_3$ $Mo$ is molybdenium. $O$ is oxygen, which his called oxide when naming compounds. Oxygen has a charge of -2. The compound is neutral, so the charge/oxidation number of the manganese element was +6 (-2*3 because there are 3 oxygen atoms in the compound). The name is written as element (oxidation number) element so $MoO_3$ is molybdenium (VI) oxide.