Salut! :)
x² + x = 2 × ( 1 + 2 + 3 + ... + 2021 )
x × ( x + 1 ) = 2 × ( 2021 × 2022 : 2 )
x × ( x + 1 ) = 2021 × 2022 => x = 2021 => H = { 2021 }