In $△$ OSP,$\angle \mathrm{OSR}={90}^{\circ }$
[Radius is perpendicular to tangent]

In $∆$ TSR
$\mathit{x}+{90}^{\circ }+{65}^{\circ }={180}^{\circ }$
(Angle sum property of triangle)
$\therefore \mathit{x}={25}^{\circ }$
$\angle \mathrm{SOQ}=2\angle \mathrm{STR}$
[Angle at centre $=2\times$ Angle at circumference]
$\mathit{y}=2\times 25={50}^{\circ }$
In $∆$ OSP,
${50}^{\circ }+{90}^{\circ }+\mathit{z}={180}^{\circ }$
(Angle sum property of triangle)
$\therefore \mathit{z}={40}^{\circ }$