a ball is thrown with ainial velocity of u=(10i+15j)m/s. when it reaches the top of its trajectory neglecting air resistance a velocity b acceleration?

I assume j is up :)

at top the horizontal velocity component is still10 since there is no vertical acceleration
however the vertical velocity is of course zero
so
a. 10 i + 0 j
b. g = a = -9.81 m/s^2 on earth
so
a = 0 i - 9.81 j
the whole time