1 = - a - 13 divided by - 6

-7 = a

Step-by-step explanation:

1 = - 1a - 13/-6

multiply - 6 on both sides

-6 = - 1a - 13

add 13 on both sides

7 = - 1a

divide - 1 on both sides

-7 = a

explanations : When you do something on one side of the equation you have to do it on the other. Why add a 1 to - a (it is totally optional). You can add a 1 to a variable if it makes the equation easier (so the last step was optional as well)