Hello, I think "adjusting" is a participle in your sentence, not a gerund. A gerund is an 'ing' form of a verb which could be used as a noun. It can function / play the role of a subject, an object, a verb complement etc.
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Mello kunThe answer in my book is "adjusting", but I don't understand why that is.This use of have, with an object such as difficulty/trouble/a hard time/a problem, allows only a gerund-participle verb-form.