Help and explanation figuring out what the value of the modified duration should be

A bank has the following balance sheet: Assets Liabilities & Equity $500 million Debt $420 million Net Worth $80 million In order to immunize the bank's Net Worth, its manager constantly manage the modified duration of the bank's Assets portfolio accordingly. If the bank's debt portfolio currently has a modified duration of 9.7 years. What should be the value of the modified duration? Round your calculations to the nearest 0.01, i.e., two decimal places