Take a look at Chapter 16, Problem 59 in your Cutnell & Johnson text.
A dish of lasagna is being heated in a microwave oven. The effective area of the lasagna that is exposed to the microwaves is 1.6×10-2 m². The mass of the lasagna is 0.25 kg, and its specific heat capacity is 3400 J/(kg•C°). The temperature rises by 80.0 C° in 7.0 minutes. What is the intensity of the microwaves in the oven?