Problem $1$: Generate a random $4$x$4$ matrix A and a random $4$x$1$ right$-$hand$-$side b$.$Use Julia to do an LU decomposition on your matrix A$.$ Solve Ax $=$b by using the backslash operator on L and U with appropriate right$-$hand$-$sides. Show the$$ work done by Julia in the output cells $($this should be automatic$).$

I can not do F $=$ lu$($A$)$ in the code because it saids:

UndefVarError: $`$a$`$ not defined

Stacktrace:

$[1]$ top$-$level scope

@ In$[16]$:$1$