Question

a-6=8-(9+a)

Answer 1

a - 6 = 8 - (9+a)
a - 6 = 8 - 9 -a

now, simplify :)

Answer 2

a - 6 = 8 - (9+a)
distribute the - sign on the right side, then simplify the 8-9 so you have
a - 6 = -1 - a
add 6 to both sides, and you have
a=5-a
so your equation has no solution

Answer 3

a - 6 = 8 - 9 -a
Combine like terms:
a - 6 = 8 - 9 -a
2a = 5
a = 5/2
a = 2.5