Dear friend, the sentence in question basically consists of two parts, namely, Subject (He) and Predicate, also referred to as a long verb phrase (is extremely helpless at maths). The latter, in turn, is further analysable into unitary and multiple constituents; they are as follows: Verb (copula is - unitary consittuent) and Adjective Phrase (extremely hopeless at maths - multiple constituent). Furthermore, the adverb (extremely), not being a clause element, operates grammatically as part of an Adjective Phrase (modifier of 'hopeless').
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