What Angle Ensures the Ball Appears to Move Straight?

[supercali]

Homework Statement

a railcart A moves in a fixed accelaration a_1=a_1 \hat{x} (a_1 is relavive to earth) at moment t=0 a ball is thrown from it in the velocity v_0 (v_0 is relative to the railcart A) and with the angle \alpha above the horizon. the velocity of the railcart when the ball was thrown was \vec{v_1}=v_1\hat{x} (v_1 is relavive to earth). (the mass of the ball is neglectable relavtively to the railcart so that the act of throwing the ball doesn't affect the railcart)
behind railcart A moves another railcart B and on it a man. railcart B moves in a fixed accelaration a_2=a_2 \hat{x} (a_2 is relavive to earth) the velocity of the railcart B when the ball was thrown was \vec{v_2}=v_2\hat{x}(v_2 is relavive to earth)
the man on railcart B sees the ball moving in a straight line. what should be \alpha for it to happen? (you can state \alpha as its tan(\alpha)

Homework Equations

The Attempt at a Solution

for the man this is true
\frac{v_y}{v_x}=\frac{F_y}{F_x}
i tried to use the galilean transformation
but i don't seem to pull it off

this question is really hard in my opinion
if you can give me a hand here

 

[supercali]

please can someone give me a hand here