Question: solve command has no answer in some cases

I write this code:

> e1 := a-3;
                             a - 3
> e2 := sqrt(6)*b+6;
                           (1/2)      
                          6      b + 6
> trac := e1+e2;
                                 (1/2)  
                        a + 3 + 6      b
> dter := e1*e2;
                             / (1/2)      \
                     (a - 3) \6      b + 6/
> dell := trac^2-4*dter;
                           2                           
         /         (1/2)  \              / (1/2)      \
         \a + 3 + 6      b/  - 4 (a - 3) \6      b + 6/
> solve(dell > 0);
            /         (1/2)  \    /     (1/2)      \ 
           { a < 9 + 6      b }, { 9 + 6      b < a }
            \                /    \                / 
> solve(dell = 0);
                   /         (1/2)         \ 
                  { a = 9 + 6      b, b = b }
                   \                       / 
> solve(dell < 0);

> solve(dell = 0);
                   /         (1/2)         \ 
                  { a = 9 + 6      b, b = b }
                   \                       / 
> solve(dell < 0);

whay does the maple give no answer in last line?