I got the answer of 432units^2
Since the length of the side and height are the same, 25, I subtracted 4 from 25, resulting in 21. To find the area of a of the missing triangle on the left side of the shape.
21 * 16, which is the length of the base results in 336/2 by using the triangle formula is 168.
The other smaller missing triangle on the right, I subtracted 15 from 25 to get the missing height of the triangle which is 10.
h=10
The base would be 10, the same length parallel to the base.
h=10 b=10
10*10=100/2 = 50
Add 168 and 50 together to get 218, the added areas of the two missing triangles.
I got the total area of the "square" by multiplying 26 by 25. L*W
26*25 = 650
Subtracted 218 from 650 to get 432.