$($Problem $2)$

$($a$)(5$ points$)$ What is Thevenin equivalent circuit?

$($b$)(5$ points$)$ What is Norton equivalent circuit?

$($c$)(5$ points$)$ What is the relationship between Thevenin and Norton equivalent circuit?

$($d$)(20$points$)$ Find the Thevenin equivalent circuit from the terminal $\backslash ($ a $\backslash )$ and $\backslash ($ b $\backslash )$ to the left?