What kind of sentence structure is this: the next day, a family of pigs arrived, and they invited their friends with them.
why?

                       A complex sentence is made up of ONLY one independent clause and AT LEAST one dependent clause. (“After the storm [dependent clause], I went outside [independent clause].) The dependent clause(s) can appear either before or after the independent clause, depending on the sentence.  

                      A compound-complex sentence is made up of MORE THAN ONE independent clause and AT LEAST ONE dependent clause. (“After the storm [dependent clause], I went outside [independent clause] and I picked flowers [independent clause].)

  A compound-complex sentence is also known as Compound sentence. 

The next day, a family of pigs arrived, and they invited their friends with them. = Compound Sentence. 
 
                                                                                       ~Zєυѕ 

substitute y = 2x - 4 to x + 2y = 12:

x + 2(2x - 4) = 12     use distributive property

x + (2)(2x) + () = 12

x + 4x - 8 = 12     add 8 to both sides

5x = 20     divide both sides by 5

x = 4

put the value of x to the first equation:

y = 2(4) - 4

y = 8 - 4

y = 4

answer: x = 4 and y = 4