how to find the equivalent load value
The picture says you should divide \$Z_{\Delta}\$  to 3 if you want to have the equivalent star load. 

One way is to transform only one delta to star.

\$Z_Y = Z_{\Delta} / 3\$

So change the right load to three \$4-3j\$ impedance star (divide \$12-9i\$ to 3).

When you transform a Delta to Star, and your network is balanced, You can simply connect the center of every star to others.

Now you have one \$120V\$ source and tow parallel loads. which are \$4-3i\$ and \$4+3i\$ in parallel. It will be equal to \$Z_T\$ :)

Then divide Source voltage to \$Z_T\$. That will be the current.

As you know you can just get one load and one source (in balanced 3phase) and unseen others.