x+1.5x=12.5
2.5x=12.5
x=5
0.4x+7.2=10
0.4x=2.8
x=7
2.1x（x-3）=2.53
2.1x-6.3=2.53
2.1x=8.83
x≈4.20
x-3.7=1.52
x=1.52+3.7=5.22