a. The resistors in figure P.3.4-2 are in series only so the sum of them form a large equivalent resistor of resistance R = 15 Ω. As the voltages are the same in fig. (a) and (b), then R = 15 Ω makes the 2 circuits equivalent.
b. The current is I = U / R = 28 / 15 = 1.866 A
c. The power supplied by the source is P = U*I = R*I*I = 28*28/15 = 52.26W