The total number of crutches at the beginning of the month = 392 sets
The total number of crutches at the end of the month = 37 sets
The number of sets of crutches that hospital used last month = Total number of crutches at the beginning of the month - total number of crutches at the end
The number of sets of crutches that hospital used last month = 392-37 = 355 sets
The number of sets of crutches that hospital used last month = 355 sets