3*(25-7х) = 12                                     60 = 2 * 2 * 3 * 5; 84 = 2 * 2 * 3 * 7

 75 - 21х = 12

21х = 75-12

21х=63

х = 63:21

х = 3                                                            НОД(60 и 84)= 2*2*3=12