'Carpenter's square' seems an odd name for something L shaped but the key to its name, as to this tough nut, is in right angles. To prove that ÐPBQ = ÐQBR = ÐRBC first draw the diagram with the carpenter's square placed so that R is on the line DE, P is on the line AB and the edge QT contains the point B. Draw the lines BQ and BR. Then draw the line RX perpendicular to BC with X on BC and look for three congruent triangles.