How this equation estimates values

Hi,

When I run "make/o/n=21 mass", there will be make vale of 0 in mass wave. But when I run "make/o/n=21 mass=p+2", then the wave mass will have values (see picture 1). Is the p has values like 0, 1...20? After then, I run "mass[15, 20]=17+5*(p-15)^2", then the wave mass will have values like picture 2. If p has values like 0,1...20, why the equation of mass[15, 20]=17+5*(p-15)^2 will calulate the values in picture 2?
yes, p is the row marker starting from zero. If you execute edit mass in your experiment a table will be displayed. The point column to the left is essentially what p will give you in wave assignments.
Run the command DisplayHelpTopic "Waveform Arithmetic and Assignment" to read more about this.
There was no picture attached so I don't know what is going on, but basically replace p with 0...20 and you should be able to check the values yourself. If something is wrong rather post an example experiment.