So, here's my attempt to put everything in order.
Consider, that we know how to draw a line and nothing more. Here's the corresponding method declaration for it:
void drawLine(int x0, int y0, int x1, int y1, Color color);
We need to implement a method which will draw a triangle and fill it with a specified color. Here's the corresponding method declaration:
void drawTriangle(Vec2 v0, Vec2 v1, Vec2 v2, Color color);
Here's the visual representation of what this method should do:
drawTriangle(v0, v1, v2, Color.BLACK)