1)
21x-126=20x+20-36x-72
21x-126=-16x-52
21x+16x=126-52
37x=74
x=74:37
x=2
-----------
2)
18x-144+35x+175=56x+12+10
53x+31=56x+22
56x-53x=31-22
3x=9
x=9:3
x= 3
----------------------
3)
18x+45-80x-112=42x-630-165
-62x-67=42x-795
42x+62x=795-67
104x=728
x=728:104
x=7