My mother acquired her knowledge of English in the United States.
Literal
Mother [topic-は] United-States [at-で] English [possessive-の] knowledge [object-を] acquired
習得する (しゅうとく) = to acquire/master (a skill or knowledge) — more formal than 学ぶ or 覚える. 米国 (べいこく) = United States (formal; casual: アメリカ). 知識 = knowledge.