Divide the volume by the length of the prism to get the area of the face:
468/13 = 36 square yards.
The area of the face ( triangle) is 1/2 x base x height
Replace with what you have:
36 = 1/2 x 9 x g
Multiply both sides by 2 to remove the 1/2:
72 = 9 x g
Divide both sides by 9 to find g:
g = 72 /9
g = 8 yards.