Idon't understand this one either i bed show your work and answer

Well well we find the area of the triangle first:
area = 1/2 * base * height
area = 1/2 * 8 1/4 * 5 3/4
area = 1/2 * 33/4 * 23/4
area = 1/2 * 759/16
area = 759 / 32
area = 23 23/32

now if we multiply the length of the triangles we get volume because the volume of the prism is really just the sum of the areas of all the triangles that make up it's length. So:
volume = 23 23/32 * 12
volume = 276 276/32
276/ 32 = 8 5/8
volume = 284 5/8