Need help!

Need help!
 

Hi, I have a question about spanish grammar.

I'm doing a first year course in spanish and I got my homework back, and I got a question wrong and I don't know why?

The question was asking what would you do if you were famous.
So my answer was I would be friends with johnny depp and leonardo di caprio.
So I wrote "Yo seria el mejor amigo de Leonardo dicaprio y Johnny depp".
My teacher said this was wrong and corrected "el" to "la" and "amigo" to amiga".

Why is this?
I thought I'd write el mejor amigo because I am referring to males.

Adjectives almost always agree with noun gender and a noun sometimes agrees with the one's sex.

If you are a male: Sería el mejor amigo...
If you are a female: Sería la mejor amiga...

Besides, it's possible to use another structure:

Me amistaría con...

A pleasure.

What Julvenzor said is right.

Your sentence has these parts:

Yo sería la mejor amiga de Leonardo DiCaprio y (de) Johnny Depp.

Yo = subject, missT, human female

sería = verb

la mejor amiga = complement of ser, identification of subject, must agree in gender and number with the subject

de Leonardo DiCaprio y (de) Johnny Depp = prepositional phrase functioning as adjective and that further describes the complement la mejor amiga