04. (B) Alternative Method for Subtract

Step 1
	If you do not have access to the Solid Tools, or you have not successfully created a "Sketchup Solid" then you will need to combine and edit two components as follows. Start off by selecting both the triangular prism group and the External Walls component. Right click and select "explode".
Step 2
	You should now have all of the objects and individual lines and faces again. Select everything using either triple click or "ctrl+a". Right click and select "Intersect -> With Selection".
Step 3
	You now need to carefully delete all of the parts of the triangular prism. I do this by choosing optimum views and drag selecting, such as here.
Step 4
	A clever drag selection gives you this.
Step 5
	Now just hit "Delete".
Step 6
	Repeat this over the bits on the inside.
Step 7
	Hit Delete.
Step 8
	So you have this.
Step 9
	Now just select the faces which cover the opening.
Step 10
	Hit delete.
Step 11
	You now have exactly the same opening as you would have had from a subtraction, it has just taken a little longer.
Step 12
	Notice that we have a few blue faces so right clicking on a white face and "orienting" faces corrects this.
Step 13
	Now just select all and make a component.