The Warrior has accuracy 10, multiplied by the accuracy factor 1 from the Short sword, giving an effective accuracy of 10: acuRoll is then a real number random in [0;10[.
The Marsupial rat has dodge 3, so defRoll is a real number random in [0;3[: The Warrior successfully hits if acuRoll is greater than or equals to defRoll.
Now, consider that rectangle:
Pick a random point in that rectangle: In the green part, acuRoll is greater than defRoll, so the hit is successful, while in the red zone, acuRoll is lesser than defRoll, so the hit misses.
The red area is 3×3/2 = 4.5 and the total area is 10×3 = 30, so the green area is 30-4.5 = 25.5, then the chance to hit is 25.5/30 = 0.85, i.e. 85 %.