Answer:
The first image. 3mm, by 4mm
To find the area of a triangle, like you would with a rectangle you would multiply the base times the height:
6 * 4 = 24
Then you will mulyiply that by 1/2:
24 * 1/2 = 12
Now for the first image, you will multiply the base by the height, and see they are equal:
3 * 4 = 12
12 = 12
Explanation:
Hope it helps! =D