Explanation:
You will be going 10.km/hr with the current and 7.5 km/hr perpindicular to it ...this forms a RIGHT triangle with legs 10.7 and 7.5....use Pyhtagorean theorem to find resultant speed ( hypotenuse)
speed^2 = 7.5 ^2 + 10.5^2
speed = 12.9 km/hr