[burzum]

If you wanted to start and stop at rest you would need to have your thrusters on for half the trip in one direction and then reverse direction for the second half.

In the simple case of a constant force (assuming a constant mass):

x(t) = x_0 + v_0t + 0.5 * a t² (x_0 and v_0 are assumed to be zero)
a = 2*x(halfway)/t² = 2*50e9 m/(3.5 days)² = 1.09 m/s²

This is about 1/9th of a g. If they had the capability to have 1 g of acceleration then the transit time would only be 56 hours (1/3 of a week).

[burzum]

Oh, I should also mention that since power equals force times velocity, it probably wouldn’t be feasible to be able to power the thrusters after the beginning of the trip so the initial forces will have to be much higher.