The figure shows two triangles. To find the the area of the shaded region, you have to substract the area of the larger triangle by the area of the smaller:
 1. Area of the larger triangle:
 A1=bxh/2
 b=5 mm
 h=2mm+4mm+6mm=12 mm
 Then:
 A1=5 mmx12mm/2
 A1=30 mm²
 2. The area of the smaller triangle, is
 A2=bxh/2
 b=3 mm
 h=4 mm
 Then, you have:
 A2=3 mmx4 mm/2
 A2=6 mm²
 3. The area of the shaded region is:
 A3=A1-A2
 A3=30 mm²-6mm²
 A3=24 mm²
 4. Therefore, the answer is: 24 mm²