The next step of the strategy involves identifying a kinematic equation which would allow you to determine the unknown quantity. There are four kinematic equations to choose from. In general, you will always choose the equation which contains the three known and the one unknown variable. In this specific case, the three known variables and the one unknown variable are vf, vi, a, and d. Thus, you will look for an equation which has these four variables listed in it. An inspection of the four equations above reveals that the equation on the top right contains all four variables.