The triangle that forms is a right-angled triangle. To work out the area, you just do the base × height, all divided by 2.
Therefore, you do 5 (the base) × 4 (the height) and you get 20.
Then you divide by 2 to finish the equation and you should find that the total area of the triangle is 10 units² (20 ÷ 2 = 10).
Hope that helps :)