x =fiction books
non fictions books = fiction books -12
non fiction = x-12
total books =26
total books = fiction + non fiction
26 = x + x-12
combine like terms
26 = 2x-12
add 12 to each side
38 = 2x
divide each side by 2
19 =x
Elliot has 19 fiction books
19-12 =7
Elliot has 7 non fiction books