Answer
(a) $A\subseteq B$, $A$ is a subset of $B$
(b) $A\subseteq B$, $A$ is a subset of $B$
Work Step by Step
Try to draw a few diagrams using $A$ and $B$, we can conclude that:
(a) if $A\cup B=B$ then $A\subseteq B$, or in words: $A$ is a subset of $B$, see figure.
(b) if $A\cap B=A$ then $A\subseteq B$, or in words: $A$ is a subset of $B$, see the same figure.
