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The correct syntax for creating a function is:

function return_value = function_name(parameter_1, parameter_2,...)

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You can be lazy like me and copy-and-paste the last two statements from your previous code. Save this file as bending_stress, which is the name that MATLAB automatically assigns the file. Thus, the function name and the file name have the same name. 
Bring up beam3.m in the MATLAB editor. Make a copy of beam3.m using Save As ...  and call the new file beam5.m. In this file, comment out the lines below since this calculation is now done within the function.

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