Imagine you cut the triangle in half. You take the one half and flip it upside down and match it up next to the other hypotenuse. You now have a rectangle.
You can see that two of the sides should be 3 yd and the other two will be 4 yd because you cut 8 yd in half. To find the area you multiply length by width.
3 yd x 4 yd = 12 yd²
