How to fix face smooth after using Boolean modifier? I am new to Blender and am struggling to fix a problem with face smooth after I cut a hole through the object using the Boolean modifier.

The image shows the problem, and I tried using the triangulate shortcut, tried the edit normal modifier, tried subdivision modifier, but could not fix it.

Could anyone here help me with this?
Switch to Edit Mode
Select the vertices you want to smooth using shift+right click
Select Smooth Vertices from the Tools menu