Select the sketch entities shown in the auxiliary view to define the cut.
Hold Ctrl and select the face shown on the base feature as the starting point.
Click Cut
on the 2D to 3D toolbar and click in the graphics area
to establish the direction of the cut.
In the Cut-Extrude PropertyManager,
click Depth 
and
select the line shown in the front view to specify the depth of the cut.
Click OK 
.
Click Hidden
Lines Visible 
on the View toolbar to see the cut.
