There are 6 orange sweets and n total sweets in a bag. The probability of picking two sweets one at a time randomly and both being orange is 1/3. Show that n^2 - n - 90 = 0

Don't panic, focus on what you do know.You know 6/n * 5/(n-1) = 1/3or 30/(n^2-n) = 1/3simplify this to reach the target expression.