Answer:
x = 14
y = -7
z = -3
but this is none of the provided answer options !
Step-by-step explanation:
it's really easy by principle. it's just some work to do.
we try to find equations to express one variable in terms of the others.
but one thing there is : your teacher made a mistake.
the right solution is x = 14, y = -7, z = -3
your teacher made a typo at d.
but the right answer should be d.
just to give you an idea how this can be done (and also to prove that there is a mistake by the teacher):
we can directly try to transform one expression into one that describes x by y and z.
and then another to describe e.g. z by y. and then solve the third one just for y. and then we get the other 2 by these other expressions.
or we can e.g. add or subtract one equation to/from another. this is one of these cases, because we can find really simple expressions that way :
we do
5x + 6y + 7z = 7
- (3x + 4y + 3z = 5)
- (2x + 2y + 3z = 5)
---------------------------
0x + 0y + z = -3
=> z = -3
3x + 4y - 3×3 = 5
3x = -4y + 14
x = (-4y + 14)/3
2×(-4y + 14)/3 + 2y - 9 = 5
(-8y + 28)/3 + 2y = 14
-8y + 28 + 6y = 42
-2y = 14
y = -7
x = (-4×-7 + 14)/3 = (28+14)/3 = 42/3 = 14
