Answer
${\bf 2.4\times 10^{10}}\;\rm m/s^2\tag{Upward}$
Work Step by Step
According to the right-hand rule, the magnetic force exerted on the antiproton is upward, and the force exerted by the electric field on the antiproton is downward.
$$\sum F=F_B-F_E=ma_y$$
Hence,
$$a_y=\dfrac{F_B-F_E}{m}$$
$$a_y=\dfrac{qvB-qE}{m}=q\left[ \dfrac{ vB-E}{m}\right]$$
Plug the known;
$$a_y =(1.6\times 10^{-19})\left[ \dfrac{ (500)(2.5)-1000}{(1.67\times 10^{-27})}\right]$$
$$a_y=\color{red}{\bf 2.4\times 10^{10}}\;\rm m/s^2\tag{Upward}$$
and since it is positive, the direction of the acceleration is upward.
