Find the values of the variables in the parallelogram.

Since $\overline{\mathrm{SA}}$ and $\overline{\mathrm{EV}}$ are opposite sides, $\overline{\mathrm{SA}}\cong \overline{\mathrm{EV}}.$ Hence,

Similarly, $\overline{\mathrm{SE}}$ and $\overline{\mathrm{AV}}$ are opposite sides. Thus, $\overline{\mathrm{SE}}\cong \overline{\mathrm{AV}}.$ It follows that

Since $a=b,$ substitute b for a in $a+b=3b-6.$

Thus, $a=b=6.$