exam Maths Puzzle Numerical Ability

I need help with the Echelon Method on this problem
x-y+z=4
x+y+z=10
2x+y-3z=7
I understand the gauss jordan method to solve this but not the echelon?
3 variables throws me o

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exam Other Question

A student wrote 69 as the average of the data: 70,59,72,35,67,78,68,81,80,and 76. By how much was the answer incorrect? Is the following trinomial a perfect square:
x^2 + 4x - 4

I thought trinomials were written in sqrt form, is this not true?? Thank you