How to make a triangle with one angle being 90° Im new to Belnder and having troubles making a triangle with one angle being 90 degrees. I'm able to make a triangle but that's the existent of what I can do.



What I tried was making a normal triangle and attaching a cube to one side of it accomplishing what I want but this means I couldn't extend out side A (reference to image above) which is my goal.

Any other methods I should try?

End goal:



(but 3D)
If the second screenshot of your desired result is the side view of the future prism, then you can dissolve geometry of the cube.  

Add a cube, in Edit mode choose Edge Select to select edges. Select one of the top edges of the cube. Press X > Dissolve Edges. 





Once the edge is dissolved the mesh becomes a prism.