x-3/8=5/12
x=3/8+5/12
x=9/24+10/24
x=19/24
x-3/7=3/21
x-3/7=1/7
x=3/7+1/7
x=4/7
1/4+x+6/7=5/4
x+=5/4 -1/4-6/7
x=1-6/7
x=1/7