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Consistent equations? watch

1. So I've got this system:

and I need to find the value of a that makes this system consistent.
I know that this has an infinite number of solutions rather than a single solution but how would I find the value of a for this to be true?
I've tried adding the equations in various combinations but nothing's working out.
2. If you eliminate x from the first two equations by subtracting multiples of the third equation, then you end up with two equations which are multiples of each other on the LHS.

Now eliminate y and z from one of those two and you will find what a has to be.

Post some working if it's still not coming out.
3. (Original post by ghostwalker)
If you eliminate x from the first two equations by subtracting multiples of the third equation, then you end up with two equations which are multiples of each other on the LHS.

Now eliminate y and z from those two and you will find what a has to be.

Post some working if it's still not coming out.
a=5?
4. (Original post by BieberFan)
a=5?
I make it 6.

Final equation was "a-21=-15"
5. (Original post by ghostwalker)
I make it 6.

Final equation was "a-21=-15"
I agree with this.

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