My text says

C'est cette porte, l¨¤. = C'est la porte, l¨¤.

I don't really get why does it have to change. Porte is feminine, does it also apply to masculin nouns?

Salut, Miche. What is the funny mark after the comma (l?..)? Is this the only example you have?

C'est cette porte, (It's this/that door...)
C'est la porte, (It's the door...)

There must be more to this exercise? It looks like a lesson in using the adjective (ce, cet, cette, ces) meaning this/that or these/those. If you had a masculine noun (like le livre) the pattern would be:
C'est ce livre... C'est le livre...
C'est cet hôpital... C'est l'hôpital...
Ce sont ces gars... Ce sont les gars.....

Mme

Oh, its suppose to be

C'est cette porte, la. (with accent)= C'est la porte, la. (with accent)
It just said that this needs special attention and I don't know why.

In French, the word "la" is the feminine definite article and "le" is the masculine definite article. When using a singular noun, you need to match the gender of the noun with the correct article.

In your example, "porte" is indeed a feminine noun, so the correct form would be "la porte" (meaning "the door").

The reason for the change in your text from "la" to "là" is not related to the gender of the noun. Rather, it is a change in the word "la" to "là," which means "there" in English.

So the corrected sentence would be:
"C'est la porte, là." (meaning "That's the door, there.")