"Ella tiene la capacidad de hacer dos cosas al mismo tiempo."
Translation:She has the capacity to do two things at the same time.
Just as an aside, both prepositions "de" and "para" are acceptable in regular Spanish uses.
"The capacity" is obviously functioning as a noun accompanied by the adjectival phrase "to do two things at the same time". As you probably know by now, whenever a phrase like this is used as an adjective it must be attached to the noun with "de"
All that being said, para is in regular usage and a native speaker would not give your remark a second glance if you were to use it.
Keeping things that simple on all of our exercises will result in us learning fewer phrases overall. We learned "poder" a while back. With this sentence we learn "capacidad" and that "has the capacity" translates literally into English.
A person can stick to foreign language phrase books if he or she is only interested in learning the simplest way to say something.
But we're not learning English; we're learning Spanish. As such, we need to take the Spanish and translate to what speakers would most likely say. Translation isn't a matter of getting something word for word, as sometimes idioms prevent that, anyhow. True translation means conveying the meaning of a phrase. Similarly, in English, it is better to replace utilizar with use than with utilize.