We move all terms to the left:
-840-(-15(-7x+7))=0
We calculate terms in parentheses: -(-15(-7x+7)), so:
-15(-7x+7)
We multiply parentheses
105x-105
Back to the equation:
-(105x-105)
We get rid of parentheses
-105x+105-840=0
We add all the numbers together, and all the variables
-105x-735=0
We move all terms containing x to the left, all other terms to the right
-105x=735
x=735/-105
x=-7
