I know this is a little late, but here are some hints.

For 6.28: Consider the discriminant. First try completing the square for the terms that only have the variable $a$. Can you factor what is left?

For 6.30: The domain is all real numbers (no things to worry about here with absolute values). For the range, we know that absolute values are always $\geq 0$. Can this help? For finding the zeros (or in general for the above) it might help to consider at least a few cases to see what happens with the function.