C = 8.5x + 6y
Rearrange the equation by making the side with x, be on the Left Hand Side (LHS)
8.5x + 6y = C
Subtract 6y from both sides of the equation
8.5x + 6y - 6y = C - 6y
8.5x = C - 6y
Divide both sides of the equation by 8.5
(8.5x/8.5) = (C - 6y)/8.5
x = (C - 6y)/8.5
I hope this helped.