-4m-7+m-2 = 6

On the left side, combine the like terms

    Combine -4m and +m as -3m
    Combine like terms -7 and -2 as -9
    So the left side is -3m-9

           -3m-9 = 6

We only want letter terms on the left, so
we get the -9 off the left side by adding
the opposite +9 to both sides:

           -3m-9 =  6
              +9   +9
           ----------
           -3m   = 15 

Next we divide both sides by the coefficient
of m, which is -3:

            = 

Cancel the -3's on the left and get just m. 
Divide 15 by -3 on the right and get -5          

              m = -5

Edwin