Answer
5a+12 = 9a-12; a=6
Work Step by Step
Between the angles C and E, there are two equations containing 'a'. By setting the two equations equal to each other, you can find the value of a.
$5a+12 = 9a-12$
Get the a on one side. To do this, subtract 5a from both sides, and add 12 to both sides.
$24= 4a$
Divide both sides by 4
$6=a$ or $a=6$
