The noun phrase that's modified by the prepositional phrase in bold is Gramma Tala's computer.
The prepositional phrase "between the sofa cushions" tells us where Gramma Tala's computer is located. It is modifying the noun "computer" by telling us more about its location.
A prepositional phrase is a group of words that begins with a preposition and ends with a noun or noun phrase. The preposition shows the relationship between the noun or noun phrase and another word in the sentence. In the sentence "Gramma Tala's computer is between the sofa cushions," the prepositional phrase is "between the sofa cushions." The preposition is "between," and the noun phrase is "the sofa cushions."
The prepositional phrase modifies the noun "computer" by telling us where the computer is located. The prepositional phrase tells us that the computer is located between the two cushions on the sofa.