Answer:
x = 72 / 85
Explanation:
x -6/7=-x/84
x + x/84 = 6/7
84/84x + 1/84x = (12*6) / (12*7)
85/84x = ( 72 ) / ( 84 )
85/84x = 72 / 84
Multiply left and right from the = sign by 84/85 and you do that so you get a division ON THE LEFT SIDE that equals as 1... See how it works:
84/85 * { 85/84 } * x = 84/85 * { ( 72 ) / ( 84 ) }
{ (84 * 85) / ( 85 * 84) } * x = ( 84 * 72 ) / ( 85 * 84 )
(7140) / ( 7140) * x =. ( 6048 ) / ( 7140)
x = 6048 / 7140
x = 72 / 85