5√6 - √216 + 3√24=5√6-5√6+3√24=3√24

(√3 + а√х) (√3 - а√х) -3=(√3)^2-(a√x)^2-3=9-a^2x-3=6-a^2x

(2√х - √3у)^2=(2√x)^2-2*2√x*√3y+(√3y)^2=4x-4√2xy+3y