Use coordination and subordination to combine the short sentences below into one sentence.
Coordination and Subordination: Combine the sentences into one. You may omit words, but do not change the meaning of the sentence
Sentence: Both Americans and the English read Shakespeare. Both groups appreciate the mysteries of Agatha Christie. Her books have been international best-sellers.
Is my answer correct?: Both Americans and the English read Shakespeare, and appreciate the mysteries of Agatha Christie, which have been international best-sellers.
Once you remove the comma following "Shakespeare" ... your answer ("Both Americans and the English read Shakespeare and appreciate the mysteries of Agatha Christie, which have been international best-sellers.") employs economy of expression by combining the first two assignment sentences by means of an elegant coordinating conjunction and between what the Americans and English read and appreciate. You add the third assigned sentence by an economical relative pronoun nonrestrictive subordinated which-clause to correctly indicate the status of Christies "mysteries" as international best sellers. Your answer is precisely one excellent way to fulfill the requirements of the assignment for coordination and subordination.
Another means, admittedly less economical and elegant, is to write: "Both Americans and the English read Shakespeare, and both groups appreciate the mysteries of Agatha Christie, whose books have been international best-sellers." In this paraphrase, the coordinating conjunction and is between two independent clauses and requires a preceding comma after "Shakespeare" while a relative pronoun nonrestrictive subordinated wh-clause (whose) joins the third element to the whole; the change from a which- to a whose-clause changes the emphasis from the "mysteries" to "Agatha Christie".
You should not have put a comma in front of "and" -- it is not used to cooridinate a 2nd independent clause, you are only joining two verbs (read and appreciate). As for the last sentence, you could probably use "whose" rather than "which" to more clearly indicate the ownership of the books.
Another suggestion for better parallelism is to change the first part of the sentence to say:
Both Americans and Englishmen read...
Even with these suggestions it is still a kind of awkward sentence. If you are required to use a subordinating and a cooridinating conjunction to fix the sentence, be sure to consult your grammar text for a list of common conjunctions. Remember that only COORIDINATING conjuctions introducing a 2nd clause need a comma in front of them, suboridinating conjuctions do not.