#### Answer

a = -1

#### Work Step by Step

8 - 2(a + 1) = 9 + a
Apply distributive property:
8 - 2(a) - 2(1) = 9 + a
Simplify:
8 - 2a - 2 = 9 + a
-2a + 6 = 9 + a
Subtract a from both sides:
-3a + 6 = 9
Subtract 6 from both sides:
-3a = 3
Divide both sides by -3:
a = -1