Can you explain this solution to find Isource (Thevenin Norton)
I can see you are struggling and, when this happens, it makes a lot of sense to redraw the circuit: -

Notice how much simpler it is; just one current source and three resistors. So, to move forward, what resistance (\$R_X\$) connects across the 15 amp current source?
Once you have calculated that resistance you can calculate the voltage across the 8 Ω resistor by multiplying the 15 amps by \$R_X\$.
Once you have that voltage (\$15\cdot R_X\$) you can calculate the current that flows through the short between &quot;a&quot; and &quot;b&quot;.