Question

find the relative clause. if it is nonrestrictive, set it off with commas. if it is restrictive, submit the text without making any changes.
when we were kids, my eldest brother whose imagination knew no limits would entertain us for hours with his fanciful stories.

Explanation:

A nonrestrictive relative clause provides extra information and is set off by commas. The clause "whose imagination knew no limits" gives additional details about "my eldest brother" and is nonrestrictive, so we add commas around it.

Answer:

When we were kids, my eldest brother, whose imagination knew no limits, would entertain us for hours with his fanciful stories.