Let me fill in the statement reasoning for you:
Statement | Reasoning
∆ABC ~ ∆RST Given
∆DEF ~ ∆RST
∠A = ∠R, ∠D = ∠R Definition of ~ ∆
∠C = ∠T, ∠F = ∠T
∠A = ∠D,∠C = ∠F Transitivity
∆ABC ~ ∆DEF AA
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