On its own, a certain tow-truck has a maximum acceleration of 3.0 m/s2. what would be the maximum acceleration when this truck was towing a bus of twice its own mass?

Let's call  the maximum acceleration of the truck alone. The force produced by the engine to accelerate the truck in this situation is

with m being the mass of the truck alone.

If we attach a bus of twice the mass of the truck, now the whole system (truck+bus) has a mass of (m+2m)=3m. And in this case, the force produced by the engine is

with a2 being the new acceleration. 
The engine is always the same, so the force produced is still the same, so we can equalize F written in the first equation and in the second equation:

and find a2, the new acceleration:

Ithink the answer would be a