Answer:
D. x=y+2z
Step-by-step explanation:
if you expand out the first equation (x) you will get: a^2+b^2+c^2+2ab+2bc+2ac
then looking at all of the choices, it can be seen that the third equation (z) needs to be multiplied by to get 2ab+2bc+2ac and then added to the second equation (y) which would result in D being the correct answer.
D. x=y+2z