drawing a picture definitely helps! since BD is 6", it is split evenly into 3 and 3. AC is the same except it splits into 4 and 4. because AC and BD are perpendicular to each other, the crossing lines form a 90 degree angle. in this case, the missing side is 5 because the sections form a pythagorean triple (3, 4, 5). you could do the pythagorean theorem (a^2 + b^2= c^2) but having some pythagorean triples memorized will help a lot. i hope this helped!