How to solve how many sweets Hanna had?

This was a GCSE question in 2015. There are n sweets in a bag. 6 of them are orange, the rest are yellow. Hanna takes a random sweet from the bag and eats it. She then takes another random sweet and eats that as well. The probability she eats 2 orange sweets is 1/3. Show that n^2-n-90=0. Firstly we would right that out in a mathematical sense: (6/n)(5/n-1)=1/3 then simplify 30/n(n-1)=1/3. Next times each side by n(n-1): 30=1/3n(n-1) then times each side by 3 so 90=n(n-1). Next, expand the brackets 90=n^2-n. Finally take 90 away from each side so n^2-n-90=0