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Sum of interior angles of ΔRST :
(x + 30)° + 3x° + 90° = 180°
4x = 60
x = 15

∠S = 3 × 15° = 45°
∠R = (15 + 30)° = 45°

Since ∠R = ∠S = 45°, then RT = ST = y

Area of ΔRST
= (1/2) × RT × ST
= (1/2) × y × y
= y²/2