what is 3 to the power -1 (mod 29)?
You can think of modular arithmetic as clock arithmetic, so a number $\displaystyle {\rm{modulo}}~
29 $ is the remainder when that number is divided by 29.
In this case we are looking for a number $\displaystyle N$ in $\displaystyle [0,28]$ such that:
$\displaystyle 3N=29 \times k +1$
for some integer $\displaystyle k$
RonL