Problem with Exercise 1 in Programming Assignment C1W2 - Gaussian Elimination

I try to solve the first exercise in assignment c1w2 (Gaussian Elimination), but can not figure out what I am doing wrong.
I need help from an instructor.

The grader give this error message:

Wrong output for test case check_matrix_1.
Expected:
[[ 1. 2. 3. 4. ]
[-0. 1. 1.3 1.7 ]
[-0. -0. 1. 2.33333333]].

Got:
[[ 1. 2. 3. 4. ]
[-0. 1. 1.3 1.7]
[-5. -0. 1. 4. ]].
3 Tests passed
1 Tests failed

I have been troubling with this for hours.
Please help me!

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