You know that the interior angles of a triangle equal 180 degrees, and you know that two of the angles are 90 and 42. Do 180-90-42, and the last angle is 48.
For the matrix

the determinant using the method of expansion by minors, expanding on the third row is:

Answer:
First, we compute the determinants of the minors:

Therefore:
I don’t know sorry but hope you get the answer
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