Answer :
-2a-9=6a+15
Move 6a to the other side. Sign changs from +6a to -6a.
-2a-6a-9=6a-6a+15
-2a-6a-9=15
-8a-9=15
Move -9 to the other side. Sign changes from -9 to +9.
-8a-9+9=15+9
-8a=15+9
-8a=24
Divide both sides by -8
-8a/-8=24/-8
a=-3
Answer: a=-3