If the diameter of a students brain is about 6 inches, and you assume it's shape is approximately a hemisphere, then what is the volume of the students brain

Well, doing 6 squared should work. Then, after you get that, (36) you multiply THAT by 3.14. After this, you should then divide your number in half, since it is a hemisphere. There you go! ;) 

I may be wrong, sorry if that is the case.

Radius of the hemisphere equals to 3 inches.

Volume of hemisphere:
V = 1/2 · 4/3 · π · r³ = 2/3 · π · 3³ = 6π ≈ 18.85 in³.

Volume of student's brain is about 18.85 cubic inches.

Yes i could be the length of the triangle