2.1÷x=0.3
x=2.1÷0.3
x=7
7.2-x=4.3
x=7.2-4.3
x=2.9
x-0.84x=0.8
0.16x=0.8
x=5
30×（x÷4）=60
x÷4 =60÷30
x÷4 =2
x =4×2
x =8
84+x=10.2
x=10.2-84
x= -73.8