Answer
y=15
AC is 24
DC is 12
Work Step by Step
The diagram shows that $\overline{AD}$ and $\overline{DC}$ are congruent, so AD=DC.
$AD+DC=AC\longrightarrow$ Segment Addition Postulate
$AD+AD=AC\longrightarrow$ substitute
$12+12=4y-36\longrightarrow$ substitute
$24=4y-36\longrightarrow$ simplify
$24+36=4y-36+36\longrightarrow$ add 36 to each side
$60=4y\longrightarrow$ simplify
$60\div4=4y\div4\longrightarrow$ divide each side by 4
$15=y\longrightarrow$ simplify
AC=$4y-36=4(15)-36=60-36=24$
DC=AD=$12$
