Help! One of our equations is missing! We describe constant acceleration motion with the variables and parameters vxi, vxf, ax, t, and xf - xi. Of the equations in Table 2.2, the first does not involve xf - xi . The second does not contain ax; the third omits vxf and the last leaves out t. So to complete the set there should be an equation not involving vxi. Derive it from the others. Use it to solve Problem 29 in one step. Discuss.