Comment je conjugué ces verbes réfléchis?
I've been practicing reflexive verbs recently and I'm not sure if I'm doing it right.

Ma voiture? je l'ai _____. (vendre)
is the answer vendu or vendue? the car's feminine so would there be an "e" at the end?

Nous nous _____ les cheveux. (laver)
would the answer be --sommes lavé or sommes lavés? I wasn't sure whether or not to form agreement with an "s" since the direct object does not precede the verb.

Please tell me why as well as the answer pretty please?

Question

Comment je conjugué ces verbes réfléchis?
I've been practicing reflexive verbs recently and I'm not sure if I'm doing it right.

Ma voiture? je l'ai _____. (vendre)
is the answer vendu or vendue? the car's feminine so would there be an "e" at the end?

Nous nous _____ les cheveux. (laver)
would the answer be -->sommes lavé or sommes lavés? I wasn't sure whether or not to form agreement with an "s" since the direct object does not precede the verb.

Please tell me why as well as the answer pretty please?

Answer

I am not native French, so I might be wrong, but I am pretty sure:
1st one = vendue (voiture is feminine, precedes the verb)
2nd one = sommes lavés (reflexive verb, so takes être, so ending changes)
Bonne chance!

Answer

PS: Vendre is not a reflexive verb.   
     This exercise is about the agreement of the past participle whether the verbs are reflexive or not.

"Je conjugue."

Answer

Wystan is right:
"Ma voiture? Je l'ai vendue": Avoir being the auxiliary  the agreement of the past participle Vendue is with the direct object pronoun L' (replacing Voiture: feminine singular) because it preceeds the verb.  
"Nous nous sommes lavés les cheveux": you have to use Etre with reflexive verbs. In this case the agreement of the past participle is with the subject of the verb (les cheveux: masculine plural).

Answer

2nd one is definitely sommes lavés.

my French is a little rusty, but doesn't the first one need a 'me' in there too? Like, "je me l'ai vendue"? because the "me/te/se/nous/vous" is the part that makes the verb reflexive...?

i hope this helped!