I can sympathize with your confusion. You might be helped if you knew what the squares meant. Is 1 square = 1 inch or 4 miles? You don't know and can't tell. So I'm going to say that 1 square is 1 unit.
Givens
The vertical line is 3 units long.
The horizontal line is 4 units long.
Formula
Area = 1/2 horizontal * vertical or
Area = 1/2 * base * height Substitute givens into this formula