3.9 Square Rotts Mod n

• In this reading I was confused on how they obtained x is congruent to plus of minus 29 in the Example. But after a while I figured out that -29 is congruent to -4 mod 11. I was trying to figure out how 29 was congruent to +4. This confused me. I also don't see how this would be useful if finding the four roots mod n is computationally equivalent to factoring n.
• It was interesting finding the congruencies of squares.