Case 3a

If child_i(x) has t-1 keys but has a left or right sibling with $\geq$ t keys, then borrow one from sibling

$\;\;\;\;\;$move key from x to child_i(x)
$\;\;\;\;\;$move key from sibling to x
$\;\;\;\;\;$move child from sibling to child_i(x)

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