Initial velocity of a partial moving with constant acceleration a = - 10j m/s^2 is u = (20i +10j) m /s its speed at t =3 is 

if v is final velocity after t seconds and u is initial velocity, we have v = u+a t  , where a is acceleration

 

hence velocity v at t = 3 s ,   v = (20i+10j) -(10j)×3  = ( 20i - 20j  )  m/s²