Multiply the value of each X-coordinate by the difference between the other Y-coordinates. For point A, you will need to calculate the value of Ax multiplied by (By -- Cy). For point B, you will need to calculate the value of Bx multiplied by (Cy -- Ay). For point C, you will need to calculate the value of Cx multiplied by (Ay -- By). For each of these calculations, Ax and Ay are the x and y coordinates of point A, Bx and By are the x and y coordinates of point B and Cx and Cy are the x and y coordinates of point C.
Add these three values together. Do not be concerned if the total of these three values is a negative number.
Divide this total by 2. The absolute value of this total is the area of the triangle, which will always be a positive number.