6:10 simplifies to 3:5. The only Pythagorean triple that includes 3 and 5 is the ratio of 3:4:5. We divided by two to get the simplified ratio of the two given sides, so 1 “part” is 2. The missing segment of the ratio is 4 “parts,” so the third side is 2*4, or 8 cm. Sorry if my explanation isn’t clear.