A little googling would have answered your question:
public class Area
{
public static double AreaPolygon(int[] x, int[] y, int noOfPoints)
{
double area = 0;
for (int i = 0; i < noOfPoints; i++)
{
area = area + ((x[i] * y[i+1]) - (x[i + 1] * y[i]))
}
area = area * 0.5;
return area;
}
public static double AreaTriangle(int b, int h)
{
return (b * h) * 0.5;
}
public static double AreaRectangle(int l, int h)
{
return (l * h);
}
}