Question 921969
You handle the same way you handle positive numbers.
29 ≡ 2 (mod 3)
-101 ≡ -2 (mod 3) 
definition of modulus
a congruent b mod m
our example is -101 congruent b mode 3, implies that
-101 -b = 3k for some integer k and therefore -101 -b / 3
-101 -(-2) / 3 = -99 / 3, says that
-101 is congruent to -2 mod 3