The company discovers that there is the linear relationship between amount of money which it spends on advertising and number of units it sells. If it spends no money on advertising it sells 100 units. For every extra $4000 spent, an extra 20 units are sold.
a) If x is a amount of money that company spends on advertising, compute the formula for y, number of units sold as the function of x. (do not use commas in formula).