According to the plot, the positions at time <em>t</em> = 0 s and <em>t</em> = 19 s are -1 m and -2 m, respectively. So the average velocity for the 19-s interval is

a ball rolling on the ground
Snell's law states: n1/n2 = Sin Ф2/Sin Ф1
But n2/n1 = 1.303 and 1.326 for red light and violet light respectively (for this case) and Ф1 = 45
Therefore,
Фr = Sin ^-1{1/1.303 *Sin 45} = 32.87°
Фv = Sin ^-1 {1/1.326* Sin 45} = 32.23°
Then,
Dispersion, Фv - Фr = 32.23 - 32.87 = -0.64°