First step is to combine like terms.
9 = 7x - 13x - 21 becomes 9 = - 6x - 21
Now add 21 to both sides to isolate our coefficient and variable.
- 6x -21 + 21 = 9 + 21
The 21s on the left cancel out.
- 6x = 30
Now divide both sides by - 6 to isolate the variable.
x = 30 / - 6
x = - 5
