Answer: He earned a lot of money to buy a house.
Step-by-step explanation:
An infinitive is meant to be used as a noun, adjective or an adverb to work with a verb. It is made up of two words which are ''to + the verb''.
For instance, in the sentences: ''I love to swim'' and ''I am going to buy a PlayStation'', the infinitives are underlined and you can see they follow the basic rule in the first paragraph of ''to + verb''.
Converting the above to an infinitive will therefore take the form:
''He earned a lot of money to buy a house.''