Activity Example 3 Solve y = b - x for x. (The goal is to get x alone on one side of the equation.) SOLUTION Since x is subtracted from b, undo that subtraction by performing the opposite operation: Add x to both sides. y = b -x y + x = b- x+x y+ x = b -yty +x=-y+b x = -y+b or x =b-y 11. Solve P = R - C for C. (Hint: Add C to each side of the equation, simplify, and con- tinue to solve, as demonstrated in Example 3.) 12. In retail sales, the relationship between an item's cost, C, the amount of the markup in price, M, and the selling price, S, is M = S - C. Suppose the selling price of an item is $21. a. Write a formula for the markup on the item. b. Solve the formula for the cost, C