First, I calculated the acceleration of A by the slope:
-0.5m / s ^ 2
in t=4/s
V=Vo+at (To bodie A)
V=4-4.0,5
V=2m/s
With that, I was able to find the acceleration of B:
0,5m/s^2SA= 10 +4t - t^2/4
SB= t^2/4
I equaled the two, finding t = 10s
so, Va=-1m/s and Vb = 5m/s --> Vb,a = 6...