There are two ways you could go about solving this.
You could divide the length of the base (6mm) by 2 and use that to find the area or you could find the area of the whole triangle using 6mm and divide that by 2.
I will use the first method I described:
base = 6/2
base = 3 mm
height = 5.2 mm
area = bh/2
area = (3 * 5.2)/2
area = 7.8 square mm
(don't forget your units)
Using the other method would look like this:
area = bh/2
b = 6
h = 5.2
area = (6 * 5.2)/2
area = 15.6 square mm
area/2 = 7.8 square mm
As you can see either method yields the same result.
Hope this helped.
Cheers and good luck,
Brian