Answer:
R = 1.5 to the northeast
C = .5 to the west
B = C + R adding vectors head to tail
Vector B will be straight north or directly across the river
B = (1.5^2 - .5^2)^1/2
B = 2^1/2 = 1.41
Adding the vectors as described shows that if the rower rows at 1.5 m/s in a northeasterly direction and the river carries the boat downstream at .5 m/s the net speed of the boat is 1.41 m/s straight north