How can i find the height of this prism? i know that because it is an oblique prism, the height is not 17.

Using the triangle made by the dotted lines
 youhave a base dimension of 8 anfd the hypotenuse ( slanted line) would be 17
 using the Pythagorean theorem solve for height"

height = sqrt(17^2 - 8^2)

height = sqrt(289 - 64)
 height = sqrt(225)

height = 15