I can draw the right angle.
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Code, edit and compile here:
\begin{document}\begin{tikzpicture}[dot/.style={fill,circle,inner sep=1.5pt}]\coordinate (Q) at (0,0);\coordinate (A) at ($(Q)+(0:4)$);\coordinate (P) at ($(Q)+(90:3)$);\draw (A) -- (Q) -- (P) -- cycle;\path (A) -- node [midway,sloped,above] {r} (P);\path (A) -- node [midway,sloped,below] {a} (Q);\path (P) -- node [midway,sloped,below] {x} (Q);***\angle {A} -- ["$\theta$",draw,,angle radius=1cm] {angle = Q--A--P}***\tkzMarkRightAngle(A,Q,P);\end{tikzpicture}\end{document}