Company a offers a starting salary of $28000 with a raise of $3000 each year, company b offers a starting salary of $36000 with a raise of $2000 per year. company c offers a starting salary of $18000. after how many years would the salaries for company a and company b be the same? how much raise per year would company c have to offer to equal the salaries of company a and company b in the year in which the salaries of those two companies are the same?