Velocity of boat with respect to ground is Vbg = (3i + 4j) and the velocity of river water moving with respect to Vrg=(-3i - 4j).
Thus, to find the velocity of the boat with respect to the water will be Vbr.
So, Vbr = Vbg - Vrg.
Vbr = (3i + 4j) - (-3i - 4j).
Vbr = 6i + 8j.