We can use the following kinematic equation to solve for the acceleration of the cart:
v_f = v_i + a*t
where v_f is the final velocity, v_i is the initial velocity, a is the acceleration, and t is the time.
Using this equation for the motion from point A to point C, we have:
6.10 m/s = 3.80 m/s + a*(4.50 s)
Solving for a, we get:
a = (6.10 m/s - 3.80 m/s) / (4.50 s) = 0.71 m/s^2
Now we can use the same equation for the motion from point B to point C, with v_i = 3.80 m/s, a = 0.71 m/s^2, and t = 1.60 s, to solve for v_f:
v_f = v_i + a*t = 3.80 m/s + 0.71 m/s^2 * 1.60 s = 4.92 m/s
Therefore, the speed of the cart at point B is 4.92 m/s.