Step by step
The question
Question: solve x − y − z = −3, x − y + z = 3 and x + 3y + z = 15.
We call them ①, ② and ③.
We want the x, y and z that make all three true.
The plan: remove one letter twice.
Then we have two equations with two letters, and we know how to solve those.
1 Remove z from ① and ②
Look at z in ① and ②: −z and z.
They are opposites, so adding removes z.
We write ① and ② under each other: x under x, y under y, z under z.
−z and z make zero.
Cross them out.
We add: the x parts, the y parts, and the numbers.
x + x = 2x.
−y − y = −2y.
−3 + 3 = 0.
Careful!
We add every part of both equations: each letter, and the numbers on the right too.
Now we have 2x − 2y = 0.
We call it ④.
It has no z.
2 Remove z from ① and ③
Look at z in ① and ③: −z and z.
They are opposites, so adding removes z.
We write ① and ③ under each other: x under x, y under y, z under z.
−z and z make zero.
Cross them out.
We add: the x parts, the y parts, and the numbers.
x + x = 2x.
−y + 3y = 2y.
−3 + 15 = 12.
Now we have 2x + 2y = 12.
We call it ⑤.
It has no z.
Now ④ and ⑤ are two equations with two letters: x and y.
We solve them like before, by elimination.
We write ④ and ⑤ again, in columns.
3 Add the equations
Look at y: −2y and 2y.
They are opposites.
Added, they make zero.
−2y and 2y make zero.
Cross them out.
We add ④ and ⑤: the x parts together, and the numbers together.
2x + 2x = 4x.
0 + 12 = 12.
Now we have 4x = 12.
4 Find x
4x means 4 times x.
We want just x.
To remove times 4, we divide by 4.
We do it on both sides, so they stay equal.
4 divided by 4 is 1.
So just x is left.
12 divided by 4 is 3.
So x = 3.
5 Find y
We put x = 3 into ④.
2 times 3 is 6.
Now we have 6 − 2y = 0.
We want y by itself.
The 6 is in the way.
To remove 6, we take away 6.
We do it on both sides, so they stay equal.
6 and − 6 make zero.
Cross them out.
0 minus 6 is −6.
Now we have −2y = −6.
−2y means −2 times y.
We want just y.
To remove times −2, we divide by −2.
We do it on both sides, so they stay equal.
−2 divided by −2 is 1.
So just y is left.
−6 divided by −2 is 3.
So y = 3.
6 Find z
We put x = 3 and y = 3 into ①, the easiest one.
3 − 3 is 0.
−z means −1 times z.
We want just z.
To remove times −1, we divide by −1.
We do it on both sides, so they stay equal.
−1 divided by −1 is 1.
So just z is left.
−3 divided by −1 is 3.
So z = 3.
So x = 3, y = 3 and z = 3, all at the same time.
We write it as (3, 3, 3).
7 Check
Let's check in every equation.
We put x = 3, y = 3, z = 3 into ①.
We get −3, the same as the right side.
Now ②.
We get 3, the same as the right side.
Now ③.
We get 15, the same as the right side.
All of them work.
So the answer is right!
Let's look back.
First: remove z from ① and ②.
Then: remove z from ① and ③.
Then: add the equations.
Then: find x.
Then: find y.
Then: find z.
Last: check.