Company a offers a starting salary of $28,000 with a raise of 3,000 each year. company b offers a starting salary of $36,000 with a raise of $2,000 per year. company c offers a starting salary of 18,000. after how many years would the salaries for companies a and b be the same? how much of a raise per year would company c have to offer to equal the salaries of companies a and b in the year in which the salaries of those two are the same?