To find the area of a triangle, you have to multiply the height and the base length, and divide that by two.
In this question, the height is 3 square units and the base length is 4 square units.
Therefore, to find the area, you must multiply 3 by 4, which is 12, and then divide this by 2, which is 6.
So the area of this triangle is 6 square units.