You wind up with a trianglular prism. Its volume is 81. The volume of the prism is the area of the end triangle times the length. you know the length is 9. The area of a triangle is $\dfrac{1}{2}bh$. You know the triangle is a 45-45-90 triangle. If you draw in the height, you know that you are cutting the 90 degree angle in half, so you get another 45-45-90 triangle with the height and half the base as equal sides in the isosceles triangle. So, you have:
$\dfrac{1}{2}bh\cdot 9 = 81$
and
$h = \dfrac{1}{2}b$
Can you solve it from here?