2/9(1.8m-5.4n)- 3/7(2.1m-4.2)
= 2\5 M - 6\5 N - 9/10M + 9\5 = 0,5M - 1.2N + 1.8
2//9(1.8m-5.4n)-3//7(2.1m-4.2)=0.4m-1.2n-3//7*(2.1m-4.2)=0.4m-1.2n-(0.9m-1.8)=0.4m-1.2n-0.9m+1.8=-0.5m-1.2n+1.8
// - черта дроби