Tom decides to begin investing some portion of his annual bonus, beginning this year with $5,000. In the first year he earns a 10% return and adds $3,500 to his investment. In the second year, his portfolio loses 5% but, sticking to his plan, he adds $500 to his portfolio. In the third year his portfolio returns 2%. What is Tom's dollar-weighted average return on his investments on three-year period?