A, 34+(9-21)=3417-(x+3417)
34 + (-12) = 3417 - x - 3417
22 = ( 3417 - 3417 ) - x
22 = 0 - x
x = 0 - 22
x = -22
Vậy: x = - 22
B, (15-x)+(x-21)=7-(-8+x)
15 - x + x - 21 = 7 + 8 - x
-x + x = 7 + 8 - x - 15 + 21
0 = 15 - x - 15 + 21
0 = ( 15 - 15 ) - x + 21
0 = 0 - x + 21
-x = 0 - 0 - 21
- x = -21
x = 21
Vậy : x = 21