Given the following circuit below:

$V_S=12V,I_{S1}=3mA,I_{S2}=1mA,R_1=4k,R_2=2k$

$($a$)$ Find the equivalent resistance $R_{TH}(10$ points$)$

$($b$)$ Find the open circuit voltage $V_{OC}(10$ points$)$

$($c$)$ Find the short circuit current ISC $(5$ points$)$

$($d$)$ Construct the Thevenin equivalent circuits $(5$ points$)$