Answer
$A\cup C=\{a,b,c,d,f,g\}$
Work Step by Step
Recall that an element $x\in A\cup C$ if and only if $x\in A$ or $x\in C$. Thus, we find the union by finding the elements that are contained in either set $A$ or set $C$. This gives us $A\cup C=\{a,b,c,d,f,g\}$.
