Answer
molybdenium (VI) oxide
+6
Work Step by Step
$MoO_3$
$Mo$ 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.
