Choose I_s such that the power absorbed by element 2 in the figure is 7 W.
We know that the power absorbed by element 2 is 7W, thus we can write:
We also know that p=vi where p is power, v is voltage and i is current.
Now, we substitute the values given and isolate for i:
This question can be found in Basic Engineering Circuit Analysis, 10th edition, chapter 1, question 1.30.