Error propagation of distance modulus and parallax

Hi!

Here is my problem: there is a star, for which we know the distance, d=21.2 pc,

the measurement error is delta_d=1.8 pc. The question is that how far should we put this star,

so that the following equation would be true: d/delta_d = 3?

The teacher told me to use two formulas: m-M=-5+5log(d) and pi=1/d (pi=parallax),

and then do some error propagation calculations.

Of course, I have already tried it using the equations above,

but I did not succeed.

Does anybody have any idea?

Actually, it would be OK as well, if anybody could do it using

any other equations.

Thank you, and sorry for my english.

Cheers, Joe