Answer: C) to learn methods for solving equations.
Explanation: An infinitive phrase is a group of words that uses an infinitive (“to” + verb), it will also include objects and modifiers. An infinitive phrase can act like a noun, adjective, or adverb. In the given sentence, the infinitive phrase is "to learn methods for solving equations" in this phrase, the infinitive is "to learn," "methods" is the direct object, and "for solving equations" is a modifier (prepositional phrase).
