When calling a function or indexing a variable, use parentheses.

Hello I am trying to solve this but I need some help to make it work. Kinda new working around matlab. your help would be totally appreciated. Thank you.

Answer 1

[DGM]

The error message tells you what to do. diag() is a function; [1 2 3] is a vector, so:

diag([1 2 3])

ans = 3×3

1 0 0 0 2 0 0 0 3

Answer 2

[Walter Roberson]

The syntax for calling functions is function name followed by ( followed by a list of parameters followed by )

You are trying to use diag[Expressions] -- which would be the kind of calling syntax that Mathematica would use, but not MATLAB.

Use diag([Expressions]) instead.