(1)2x-20=4,
2x-20+20=4+20,
2x=24,
2x÷2=24÷2,
x=12;
(2)2x+2.8×2=10.4,
2x+5.6-5.6=10.4-5.6,
2x=4.8,
2x÷2=4.8÷2,
x=2.4;
(3)x+2.4x=5.1,
3.4x=5.1,
3.4x÷3.4=5.1÷3.4,
x=1.5.