Need help finding my mistake using Thenevin's Theorem for this circuit (not homework!)
Calculation mistake at Step number 4. You wrote:

I2 = 24/15K = 8/3K A = 2.667 mA


But 24/15k = 8/5k not 8/3k. So I2 is

I2 = 24/15k = 8/5k A = 1.6mA


You will get Vth = 25.6 V.