The area of a triangle is (base × height) / 2. The base can be any side, and the height is the perpendicular distance from that side to the opposite vertex.
A triangle with base 6 and height 4 has area (6 × 4) / 2 = 12.
When given three vertices as coordinates, you can use the shoelace formula or Heron's formula. For simple right triangles, treat the two legs as base and height.