you can search in google if you want for the "inscribed quadrilateral conjecture"
and if you want... I can give you a quick proof of it
hmmm in short, if you inscribe a quadrilateral polygon in a circle
it will have 4 angles
each pair of opposite angles, are "supplementary angles", or they add up to 180°
so.. that said in yours, R+P = 180 and Q+O = 180 as well
you only need Q
so [tex]\bf \measuredangle Q+\measuredangle O=180\qquad
\begin{cases}
\measuredangle Q=2x+4\\\\
\measuredangle O=2x
\end{cases}\implies (2x+4)+(2x)=180[/tex]
solve for "x"
how big is Q? well, 2x + 4 :)
