Since AM is a median, then M is the midpoint of BC. Therefore, BM=MC=6.
Similarly, since BN is a median, then N is the midpoint of AC. Therefore, AN=NC=7.
By the Pythagorean Theorem on right triangle ABM, AB2=AM2+BM2=72+62=85. Then, AB=85.
Similarly, by the Pythagorean Theorem on right triangle ACN, AC2=AN2+NC2=72+72=98. Then, AC=98.
Finally, by the Pythagorean Theorem on right triangle ABC, BC^2 = AB^2 + AC^2 = 85 + 98 = 183.
Therefore, the length of the hypotenuse is BC = sqrt(183).