。
His analysis of the cause of the riot was correct.
Literal
Riot [possessive-の] cause [about-についての] his analysis [topic-は] was-correct.
について + の creates an attributive phrase: 'the analysis about the cause.' The の chain (暴動の原因についての彼の分析) shows how Japanese stacks modifiers left of the head noun, building up a complex noun phrase without relative clauses. 正しかった is past tense of the i-adjective 正しい.