All you need is the double angle identity:

So we have

Apply the identity again to the squared term:

Answer:
a = -1
Step-by-step explanation:
-6(3a+4) = 3a-3
-18a-24 = 3a-3
-24+3 = 3a+18a
21a = -21
a = -21 ÷ 21
a = -1
2 and 8 are alternate exterior angles