2/7m-1/7=3/14
make the denominators equal and to this just multiply the denominators of the first two equations by 2 but you have to multiply the numerators too and by the same value
4/14m - 2/14 = 3/14
with the denominators equal, you can delete them
4m-2=3
4m=3+2
4m=5
m=5/4