The mnemonic SOH CAH TOA helps you remember that ...
... Sin(35°) = Opposite/Hypotenuse = x/(15 cm)
... x = (15 cm)·sin(35°) ≈ 8.6 cm
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It can also help you remember ...
... Cos(35°) = Adjacent/Hypotenuse = y/(15 cm)
... y = (15 cm)·cos(35°) ≈ 12.3 cm