If $AOBC$  is a rectangle whose three vertices are $A\left(0,3\right),O\left(0,0\right)$  and $B\left(5,0\right)$  then the length of its diagonal $OC$  is

Given three vertices of rectangle are $A\left(0,3\right),O\left(0,0\right)$  and $B\left(5,0\right)$

$AB$  and $OC$  are two diagonals lengths are equal

$AB=OC$

Diagonal $OC=AB=\sqrt{{\left(x_2-x_1\right)}^2+{\left(y_2-y_1\right)}^2}$

$=\sqrt{{\left(5-0\right)}^2+{\left(0-3\right)}^2}$

$=\sqrt{25+9}$

$=\sqrt{34}$ unit