Since we're trying to express the cost, c, we know that C will be equal to something. Then, since we are buying N slices of pizza at $3 each, we have to multiply $3 by n to figure out how much the slices of pizza would cost. You multiply N by 3 because N is the number of slices, and 3 is the base cost, so then by multiplying them, we find out how much it would cost for any number of slices, n. This gives us the equation c=3.00n, which is the same as c.