Now let's return to our destroyer problem. What we want is a function called
velocity that takes time as an argument and returns the velocity of the
destroyer at that time. Since we are describing the x and y coordinates of the
destroyer's position separately, what is the natural way to describe the
velocity of the destroyer?

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We can describe the velocity of the destroyer by giving its velocity in the x
direction and its velocity in the y direction. (In other words, since we are
giving the position of the destroyer as a vector, we can naturally give its
velocity as a vector as well.)