How we cite our quotes:
(Orestes): "O Zeus! Grant me vengeance for my father's death! Be my ally if you will!" (18-19)
The king of the gods, Zeus was also in charge of dealing out Justice. Thus, when Orestes asks Zeus to let him take vengeance, he basically means that he wants Zeus to ensure that his act of vengeance is just.
(Chorus): "The bloodshed drunk up by Earth its nurse –
the vengeful blood is set hard, and it will not dissolve." (66-67)
These words are spoken by the Chorus of slave women at the tomb of Agamemnon. What do you think they mean here by the "vengeful blood" that won't dissolve? Remember the dramatic situation of the play, which begins with Agamemnon long-dead, murdered, and unavenged. Could they be saying that the crime of his murder (metaphorically represented by the blood) won't go away until he is avenged? You also might want to consider the metaphorical meaning of "blood" in terms of family relations. Could that have anything to do with the situation here, in which Agamemnon's son, Orestes, is in charge of avenging him?
(Electra): "How am I to speak sensibly to my father, how am I to pray to him? Am I to say that I bring [these mourning-libations] to a dear husband from a dear wife, from my mother? I have no words for that, no words I should say as I pour this offering on my father's tomb. Or am I to follow men's custom and make my speech this, that he should well repay those who send these offerings, and with a gift which their goodness deserves?" (88-93)
Nothing about revenge here, is there? Well, maybe, maybe not. What do you think Electra means when she considers asking that Agamemnon "well repay those who send these offerings, and with a gift which their goodness deserves"? Let's bear in mind that it was Clytemnestra, Agamemnon's murderer, who sent Electra to deliver the offerings. What sort of "gift" would be equivalent to the "goodness" of murder? If you follow through on this line of thinking, you can see that what Electra might really be saying here is that she wants Agamemnon's spirit to bring vengeance upon Clytemnestra.