Back fill with sand or gravel. Compact it. Do not put dirt back in cause it won't compact well enough and your slab will sink. Get a piece of 6x6 remesh for the size of the slab and then order a cement truck. Pay the extra for a short load. Your back will appreciate it and mixing 33 bags is no fun. Form up some other stuff such as paving stones, splash blocks, etc or contact a neighbor who might want some of the extra concrete from a load such as for fence posts. Good luck.