"Nein, die Spiegel gehören einfach dazu."
I find the dictionary hints terribly unhelpful in these cases.
The verb is 'dazugehören' (= to belong to it, to go with, to be among), it's conjugated like this:
ich gehöre dazu
du gehörst dazu
er/sie/es gehört dazu
wir gehören dazu
ihr gehört dazu
sie gehören dazu
The dictionary hint however is only for 'gehören' (=to belong) without giving an indication that 'dazu' is a part of the verb. And then it's translated as 'are fitting', 'be fitting', which is not only wrong, but also doesn't help with this sentence - hence the confusion in the comments.
This could be a context: [at IKEA] "Hmm, darling, I love this cabinet, but it looks much nicer with the mirrors. How much are those? Are they sold separately?" - "No, the mirrors just come with it."
I think the original German sentence could also be translated literally, like "No, the mirrors just belong to it."