Answered
I cannot get the Length(<Function>, <Start Point>, <End Point>) command to work. For example (v 6.0.609.0 offline):
I know that points C and D are on function f, yet when I input the command, I get a "Sorry, something went wrong. Check your input." error message. I know the Distance command is an alternative. It is my (albeit noob) understanding that command will not measure along function f, it will calculate the shortest distance. I'd prefer a command that reassures my students it measured along f since that was a specified parameter.
Thank you for your time.
The same question 
In your file you defined f as a line and not as a function, that's all. Even though the graph of a linear function is a line defining it as a function makes it a function, defining it as a line between two given points makes it a line and commands can only be applied to certain types of objects. E.g. you can't calculate the derivate of a line and you can't calculate the slope of a function.
for line(A,B) use distance(A,B)
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