I need help please on each of the bullet points.

i am positive it is c. condensing but i may be wrong

try using substitution:

x + y - 6 = x - y. (i'm not giving you the exact answer). since 0 = x - y, then sub 0 in the equation with x - y. combine your like terms, and then use inverse operations, and you'll have your answer.

: -)

b.have there own function.

carbon atoms, hydrogen, oxygen, and nitrogen.