Since a stream of water is perpendicular to the movement of Becca, her boat must have been moving going at an angle of north east. Using the Pythagorean formula, we can calculate for the resultant velocity, c.
c^2 = (8 mph)^2 + (6 mph)^2
c^2 = 64 + 36
c^2 = 100
c = 10 mph