From Mechanics

In this case we have \displaystyle \mathbf{u}=5\mathbf{i} and \displaystyle \mathbf{a}=-4\mathbf{j}

Substituting these into the equation \displaystyle \mathbf{v}=\mathbf{u}+\mathbf{a}t \ \ gives the velocity of the ball at time \displaystyle t=4 \text{ s } as:

\displaystyle \mathbf{v}=5\mathbf{i}+(-4\mathbf{j})\times 4

which gives

\displaystyle \mathbf{v}=5\mathbf{i}-16\mathbf{j} \ \text{ m}{{\text{s}}^{\text{-1}}}