Inactive Tutor answered 05/16/13
When working with equations such as this one you need to get all the variables(letters) on one side and the numbers on the other. You do this by adding the opposite of the value to both sides. so...
7-6a = 6-7a
-6 = -6
1-6a= -7a
+6a= +6a
1 = -1a
-1 = a
To check this you would use -1 instead of a
7-6(-1) = 6-7(-1)
7+6 = 6+7
13 = 13