We know that f(f(x)) = x for all x.  If x < 2, then f(x) = ax + b.  Substituting into the first equation, we get
 f(ax + b) = x
 
 Since f(ax + b) = 8 - 3(ax + b) if ax + b >= 2, then we must have ax + b >= 2.  This means that x >= -2/a.
 If -2/a <= x < 2, then f(x) = ax + b.  Substituting into the first equation, we get
 (ax + b) + b = x
 
 This simplifies to ax + 2b = x.  Since ax + b >= 2, then x must equal 2.  However, we know that f(2) = 8 - 3(2) = 2, so this case is not possible.
 Therefore, the only possible value of x is x = -2/a.  In this case, f(x) = f(-2/a) = 8 - 3(-2/a) = 10/a.  Substituting into the first equation, we get
 10/a + b = -2/a
 This simplifies to a + b = -10.  Therefore, a + b =  −10