خَلْفَكَ = khalfa is an accusative place adverbial and first member of iḍāfa; -ka is genitive possessive pronoun.
أَمَامَكَ = the same structure with amāma.
The attached “ka” is not a direct object of the verb.
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Structure of Khalfaka and Amāmaka
Al-Mafʿūl Fīhi: Adverbials of Time and Place
خَلْفَكَ = khalfa is an accusative place adverbial and first member of iḍāfa; -ka is genitive possessive pronoun.
أَمَامَكَ = the same structure with amāma.
The attached “ka” is not a direct object of the verb.