You need to figure out the total area of the pizza (a circle) and then divide it by the area of each slice to get the number of slices of pizza. By the way, area should be in square units, so I assume you mean 14.125 in2.
So, Area of a circle = πr2
Diameter = 2 x the radius (r). So, if the diameter = 14 in, 14 in = 2 r, and dividing both side by 2, r = 7 in.
Area of a circle = Π (7 in)2 = 153.938 in2
Now, divide this total area of the pizza, 153.938 in2, by the area of one slice, 14.125 in2.
153.938 in2/14.125 in2 = 10.898 slices or about 11 slices of pizza