1. Substitute X=2, Y= 1, and get: 2*2- 1=M, m = 3,; 2+M* 1=N, substitute M=3, N=5, so m-n =-2;

2. Substitute X=2 into the equation and get: 3*2+k/2- 1=2+3*k, 3=5K/2, and K=6/5.

3. Substitute K=2X+3Y into 3X+5Y=K+2 to get 3X+5Y=2X+3Y+2, X+2Y=2, plus the conditions X+Y= 12, Y= 12-X, y =/kloc-0.

4. 1.X/3-X- 1/6= 1，-2X/3=7/6，X=-7/4

2.X+2/3=X/5，-4X/5=2/3，X=-5/6

3.X-2Y=7 equation ①, 4y=3/2 equation ②, Y=6 from equation ②, and substitute Y=6 into X-2Y=7, so X= 19.

Ask yourself the fourth question. You can't know nothing. The method is the same.