notice the tickmarks
AD = DC
now AC, which is the whole length from side to side,
is 4y-36
AD is 12, and since DC is the same as AD, so
DC = 12
AC = AD + DC, or 12 + 12
now, we know AC is also 4y-36
so [tex]\bf \begin{array}{cccccclllllll}
AC&=&AD&+&DC\\
\downarrow &=&\downarrow &&\downarrow \\
4y-36&=&12&+&12\\
\\ \quad \\ \quad \\
4y-36&=&&24
\end{array}[/tex]
solve for "y"
