Since $225=3^2 5^2$, the prime ideals correspond to its two distinct prime divisors, giving x = 2. The number of units is Euler’s totient $\varphi(225)=225(1-1/3)(1-1/5)=120$. “Nontrivial” here excludes the identity unit 1, leaving y = 119. Thus x + y = 121, matching the source’s convention.
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