You can create a cut on a shellable prism.
Click Cut
.
The Cut dialog box
appears.

Select the shellable prism as the feature you want to
cut.

Select two extrusions and one plane as the cutting elements.
Click both arrows on the surfaces to reverse the direction
indicating which portion of material is to be kept. The two arrows must
point toward the shellable prism as shown here:

The Fillet radius option has changed to
Intersection radius.
The Intersection Fillet option sets the radius of fillets
on the edges produced by the cutting surface. For this cut, select
Intersection radius and enter 6mm in the dedicated box.
The Wall option lets you set if,
or how, the Cut command creates walls along the cutting surface.
We can leave the default thickness here (use feature thickness),
we will show this option in Create a Cut on
a Pocket.
Click Preview to see the result.

An offset distance from the selected cutting elements
can be entered as an option. Enter 10mm for Offset and click
Preview.

Set the offset value back to 0mm.
Click OK to confirm.
The first cut feature is created. Cut.X is added
to the specification tree in the Solid Functional Set.X
node. The 3D shape now looks like this:

Just hide both cutting surfaces if you want to see the filleted
edges.