Well, I've never seen "reversely" proportional. In fact I don't believe that "reversely" is a word in the English language! I suspect this is a mis-translation of "inversely proportional" which means just what you say: a multiple of .

" "isthe density function. While it is not a standard notation, since it is the only function given, I am sure that must be the case- and I suspect that previous examples in your text have used " " to mean the density function. You are told two things:

1) and

2)

You can use that to find k and so find the full density function.

Of course, the mass is just the density function integrated over the curve.