Equivalent circuits is often the hardest concept and most numerically intensive in the course. Learning them well could make a difference on your midterm exam. Look in several books until you find the explanation you understand best.
Since Ohm's law and Kirchoff's equations are linear, we can replace any DC circuit by a simplified circuit. Just like a combination of resistors and Ohm's law could give an equivalent resistor, a combination of circuit elements and Kirchoff's laws can give an equivalent circuit. Two possibilities are shown in figure 1.8.