Full solution of $OA$

Question

I have a geometry problem that asks for the length of $OA$ if $PR = 7cm$. I already got $7$ $cm$ as the answer, but I am not satisfied because the book didn't provide a step by step solution. I need someone to expand the answer.

Answer 1

The diagonals in rectangle $OPQR$ are equal, therefore $OQ=PR=7$ and also $OQ$ is radius too, thus $OA=7$

Answer 2

Recognize diagonals and radii are all equal

$$ OQ=PR= OA=OB = \sqrt {OP^2+PQ^2} =\sqrt {OR^2+QR^2} $$