That reminds me, I have to go check if my kid's tent is ready for camp.
The volume of this triangular prism tent will be the area of its base times its length, six feet here. The area of the triangle is of course half the base times the height.
If we call the height h we get
36 = (1/2) (4)(h)(6)
36 = 12 h
h = 3 feet
Most folks can sit in a 3 foot tall space, but it's a little tight, especially with your head squeezed up there near the apex.