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.