Overwhelmed: Module 5: Part 1: Word to Algebra Part 2

Hello, under “words to algebra” exercise 2, question 5, I received the answer of “90” after squaring the roots and subsequent 18 and 12 and solving for either y or x using elimination. However, reviewing how I missed the problem based on what was written in the “solution” box does make sense. As I have missed the problem, I do not understand exactly how my method of squaring the roots first, is not correct. When plugging in the solution of 324 and 90 respective to x and y, the equation of both simplified and expanded are still 18 and 12. Did i just overcomplicate it for no reason?

Can you provide context about what you’re talking about?