Verified Answer :

ialeccc

brain dump.... 
have you ever heard the expression cave in ?  when u see something (cave in) it looks like crater right? 
so just like that you know to draw to crater like objects perpendicular from each other 
perpendicular means at an angle of 90° to a given line, plane, or surface.

theplanesmaster

Refer to the rough drawing I have attached.
Sides 1 and 2 are perpendicular.
Sides 4 and 5 create a concave angle.

View image theplanesmaster