Ronald D. Johnson is a former employee of International Business Machines Corporation (IBM). As part of a downsizing effort, IBM discharged Johnson. In exchange for an enhanced severance package, Johnson signed a written release and covenant not to sue IBM. IBM’s downsizing plan provided that surplus personnel were eligible to receive benefits, including outplacement assistance, career counseling, job retraining, and an enhanced separation allowance. These employees were eligible, at IBM’s discretion, to receive a separation allowance of two weeks’ pay. However, employees who signed a release could be eligible for an enhanced severance allowance equal to one week’s pay for each six months of accumulated service with a maximum of twenty-six weeks’ pay. Surplus employees could also apply for alternate, generally lower-paying, manufacturing positions. Johnson opted for the release and received the maximum twenty-six weeks’ pay. He then alleged, among other claims, that IBM subjected him to economic duress when he signed the release and covenant-not-to-sue, and he sought to rescind both. What will Johnson need to show in order to prove his cause of action?