I need help editing and completing the C program below that is missing the implementation for the 'get_result' function. It needs to run as a simple calculator in C. For example:

> ./calc "1 + 2"

1 + 2 = 3

Note that you can't use the '-' operator directly in 'get_result'. 'get-result' must call 'subtract', and subtract must implement subtraction without directly using the '-' operator. Program must be able to compile!