Problem $1.$ A uniform beam of a thin$-$walled angle section as shown the Figure $5\mathrm{.}29$ below is subjected to

the bending M$\_($y$)($M$\_($z$)=0).$ Find the neutral axis and bending stress distribution over the cross$-$section.

Figure $5\mathrm{.}29$ Thin$-$walled angle section

Important: please explain clearly what each value we plug in the centroids equation represents and how we get it$!!!!$

i have the solutions but I don't understand how we get these values so please explain the concept