Since PS = RS, the triangle PRS is isosceles. This means the angles SPR and SRP are congruent.
The interior angles of any triangle sum to 180° in measure. So we have
∠SPR + ∠SRP + 90° = 180°
2 ∠SPR + 90° = 180°
2 ∠SPR = 90°
∠SPR = a = 45°
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