Given the task definition and input, reply with output. This task is about creating an unanswerable question based on a given passage. Construct a question that looks relevant to the given context but is unanswerable. Following are a few suggestions about how to create unanswerable questions:
(i) create questions which require satisfying a constraint that is not mentioned in the passage
(ii) create questions which require information beyond what is provided in the passage in order to answer
(iii) replace an existing entity, number, date mentioned in the passage with other entity, number, date and use it in the question
(iv) create a question which is answerable from the passage and then replace one or two words by their antonyms or insert/remove negation words to make it unanswerable.

Passage: In 1986, Michael Dell brought in Lee Walker, a 51-year-old venture capitalist, as president and chief operating officer, to serve as Michael's mentor and implement Michael's ideas for growing the company. Walker was also instrumental in recruiting members to the board of directors when the company went public in 1988. Walker retired in 1990 due to health, and Michael Dell hired Morton Meyerson, former CEO and president of Electronic Data Systems to transform the company from a fast-growing medium-sized firm into a billion-dollar enterprise.
What year didn't Michael Dell bring in Lee Walker to the company?