[tex]81 - x \: \\ belenoe \: na \sqrt{y} + 9 [/tex]

81-x/√y+9=(81-x)√y+9/√y+9*√y+9=(81-x)√y+9/y+9

(81-x)/(√y+9)=(81-x)(√y-9)=(√y+9)(√y-9)=(81-x)(√y-9)/(y-81)