In$-$class exercise: backward substitution

Write a function that implements backward substitution. Please follow the requirements below for full credit:

Include the function in the file

linsolve.py$.$

The first line of the function definition should be def back$\_$sub$($lu$\_$fac, y$)$:

where lu$\_$fac is $2d$ NumPy array and y is a $1d$ NumPy array.

lu$\_$fac can be assumed to be the $LU$ decomposition of some matrix, with $U$ in the upper part.

back$\_$sub should return x$,$ a Id NumPy array, that holds the solution to Ux $=y.$

Include a docstring in the function and use $-=.$

$1$