Hi!
The area of the shaded region is
The area of the whole triangle - the area of the smaller triangle
So first let's find the area of the whole triangle.
a = 0.5bh
a = 0.5·5·12
a = 2.5·12
a = 30
Now find the area of the smaller triangle.
a = 0.5bh
a = 0.5·3·4
a = 1.5·4
a = 6
30 - 6 = 24
The answer is 24mm
Hope this helps! :)
-Peredhel