Why are you struggling with this. Show your work.
What's the area of a circle? You've half of that.
What's the relationship between the hypotenuse and the legs of an isosceles right triangle? You'll need that.
What's the area of a triangle? You'll need that.
Once the area of the cross section is known, multiply by the length and be done.
One more thing, you may wish to get your units a little more consistent. m and mm mix pretty well, but a little work is required.