please help me, step by step. Thank you!

b. What is the maximum amount of time that it would take a computer that can test 21012 keys per second to carry out a brute-force attack on a file encrypted using 256-bit AES? Give your answer in seconds and years, using scientific notation, correct to 2 significant figures. You may assume a year is 3.2107 seconds and you may use a calculator to help work out the answer. Show all your working clearly in a way that will help your tutor understand how you arrived at your