What kind of mistake was it? If the function is undefined, whether it is contained in the folder of the main function must be in the same path as the main function.

If it is a variable double, I suggest you set a breakpoint at the place where you enter the function, and then run there one step at a time, stop and look at the current values of each variable in the main window of MATLAb to see if it is a double.

If not, ask the teacher. It's hard to judge a problem with only one wrong hint.