Assume that a companys planned level of activity was 3,500 units and its actual level of activity was 4,200 units. The spending variance for one of its mixed expenses was $900 favorable and its activity variance was $200 unfavorable. The total fixed and variable portions of this mixed cost included in the planning budget was $10,000 and $2,100, respectively. What is the cost formula for the variable portion of the mixed cost that the company uses for budgeting purposes