13 = -3x-8

We want terms with letters on the left and terms without letters on the right.

So we begin by getting rid of the 13 on the left, since 13 has no letter:

We add -13 to both sides:

   13 = -3x-8
  -13      -13
---------------
    0 = -3x-21

Next we get rid of the -3x on the right since it has a letter:

We add 3x to both sides:

    0 = -3x-21
   3x    3x
---------------
   3x =    -21

Finally we divide both sides by the coefficient of x
which is 3:

    =    

Cancel the 3's on the left and divide on the right:

    =    -7

   x = -7

To check, substitute (-7) in place of x in the original equation:

    13 = -3x-8
    13 = -3(-7)-8
    13 = 21-8
    13 = 13   <--- both sides equal so it is correct.

Edwin