-2
Replace u by -2, and check if the equality remains
1-9u = 19
1-9(-2) = 19
1-(-18)=19
1+18 = 19
19=19
-2 YES
7
1- 9(7) = 19
1-63 = 19
-62=19
NO
0
1-9(0) = 19
1= 19
6
1-9(6) = 19
1-54=19
-53=19
1.6m questions
2.0m answers