Which part of speech is not appropriate in formal writing, but often used in daily conversation?
a. conjunction
b. preposition
c. interjection
d. adverb

The correct answer is C. interjection

Interjections are words loaded with emotion - when you are using them, their meaning is quite clear. For example: Ouch! Yikes! Oh no! Boo!, etc. These are not used in formal writing, whereas it is quite difficult to write a normal sentence without the use of conjunctions (and, or, when, etc), prepositions (in, with, under, etc.), or adverbs (quickly, smoothly, etc.).

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

i'm on the same lesson : / i really don't want to do it..