What I was doing was first finding the current in the circuit by combining the two voltage sources and then dividing by the 3.75 Ω resistance. From this, I used the power (supplied) equation:

Power delivered by 7 V source = ((7 v + 2.25 V) / 3.75 Ω) * (7 V) = 17.27 W.

However, the solution uses superposition to calculate the 7 V source's contribution to the power delivered:

This seems wrong to me because I thought superposition could only be used for calculating linear responses, as stated in my textbook:

So is the solution correct and, if so, what is my error with relation to the quote from my textbook?We must constantly be aware of the limitations of superposition. It is applicable only to linear responses, and thus the most common nonlinear response—power—is not subject to superposition.