REPLACE don't repair. Not a big job, even if you have never done your own concrete before. In my area, a rental mixer is $20/day and 80# bags of pre-mixed cement are $3.55/bag. A two man job, easily done in a day. What's the size of the area you wish to replace??

There are stronger and quicker drying cement formulas that will hold up and do not need to be a minimum of 4 inches to withstand pressures of a vehicle. Unfortunately the people you hired used the wrong material. We have a few different formulations to play with. Most are either polymer modifyed or cementitious urethane systems that are 5 to 10 times stronger than concrete alone. You need to find a seamless flooring systems company or concrete restoration company. Masons usually do not work witht this kind of materials.