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We learned at school that the square root of 9 is 3.
Literal
We [topic-は] 9 [attributive-の] square-root [topic-は] 3 [is-だ] [quotative-と] school [at-で] learned.
An embedded statement: the entire clause ９の平方根は３だ ('the square root of 9 is 3') is the content of what they learned, marked by と (quotative) + 習う ('learn, be taught'). Note the two は: the outer one topicalizes 僕ら, the inner one topicalizes 9の平方根 inside the embedded clause. で marks the location of learning.