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Verified answer

  9 -a    = (3-√a)(3+√a) = 3 - √a
3 + √a            3+√a

Vicus44ea (9-а)/(3+√а)= (9-а)(3-√а)/(3+√а)(3-√а)=(9-а)(3-√а)/(3²-(√а)²)= (9-а)(3-√а)/(9-а)=3-√а.