解：（1）3×7+4x=25          

                    21+4x=25      

            21+4x﹣21=25﹣21     

                      4x÷4=4÷4          

                            x=1解：（2）6x+18=48

         6x+18﹣18=48﹣18   

                   6x÷6=30÷6      

                         x=5

解：（3）1.2x﹣0.9x=8.7，      

                              0.3x=8.7，    

                      0.3x÷0.3=8.7÷0.3，        

                                  x=29解：（4）9x﹣1.8=2.7，

           9x﹣1.8+1.8=2.7+1.8，   

                       9x÷9=4.5÷9，    

                              x=0.5．