For determining V_{ Th } ,

Step 1: Open 5 kΩ load to obtain the following circuit

Step 2: Calculate the open circuit voltage across AB, which is obtained as V_{ Th }=12  V .
For determining R_{ Th } ,

Step 1: Make all active sources equal to zero.

R_{ Th }=12  k \Omega|| 4  k \Omega \text { in series with } 8  k \Omega=11  k \Omega

Now, by the Thevenin theorem, new circuit looks as follows:

So, I_{ L }=\frac{V_{ Th }}{R_{ Th }+R_{ L }}=\frac{12}{11+5}=0.75  mA

V_{ L }=I_{ L } \times R_{ L }=0.75  mA \times 5  k \Omega=3.75   V