The first step is to multiply 90 x 6 = 540, this is because the data should have 6 terms and since the mean is 90 all terms should add to 540, then add the terms you already have 92 +201+54+43+13= 403 but you need to have 540 so the last number is the difference 540 - 403 = 137
X=15. This is the Altitude theorem which is Altitude/Part = Altitude/Other part. In this question you would set up the proportion 9/X = X/25 and multiply cross products to get 225=X^2. Then you undo X^2 by doing the square route of 225 and then you get 15 as the final answer.
Your answer is 4x^2+7x-10
