The complete question in the attached figure
we know that
triangle OBR is a right triangle
so
<span>applying the Pythagorean theorem
</span>OB²=OR²+RB²----> OB²=3²+4²-----> 25
OB=√25-----> OB=5 units
the answer is
OB=5 units
we know that
triangle OBR is a right triangle
so
<span>applying the Pythagorean theorem
</span>OB²=OR²+RB²----> OB²=3²+4²-----> 25
OB=√25-----> OB=5 units
the answer is
OB=5 units