please include the quadratic functions

Solve a Problem Involving a Minimum or a Maximum Rachel and Ken are knitting scarves to sell at the craft show. The wool for each scarf costs $6. They were planning to sell the scarves for $10 each, the same as last year when they sold 40 scarves. However, they know that if they raise the price, they will be able to make more profit, even if they end up selling fewer scarves. They have been told that for every 50c increase in the price, they can expect to sell four fewer scarves. What selling price will maximize their profit and what will the profit be