There are a few ways to find the area of one slice. They all depend on what information you know.
You can do this method if you know the area of the entire pizza. Since you know the pizza is cut into 12 parts, we can assume that they are all equal and divide the total area of the pizza by 12.
If we know the dimensions of the pizza, one can use this method. Since one slice of pizza is a triangle, we can use the formula for a triangle (b * h/2) to find the area of one slice.