4/7x-1/2=6/7×3/4
4/7x-1/2=9/14
4/7x=9/14+1/2
4/7x=16/14
x=2
10X+6×（15-x）=138
10x+6 ×15-6x=138
10x+90-6x=138
10x-6x=138-90
4x=48
x=12