(5√81:√9)-(3√24:√12)=

(5.9:3)-(3.2√6:2√3)
=15-(6√6:2√3)
=15-3√2

(5√81 ÷ √9) - (3√24 ÷ √12)
= (5•9 ÷ 3) - (3√4•6 ÷ √4•3)
= (5•9 ÷ 3) - (3•2√6 ÷ 2√3)
= (45 ÷ 3) - (6√6 ÷ 2√3)
= 15 - 3√2