We first split ABC into two right-angled triangles. We name the midpoint of AC, M. AM=1cm, and BM=sqrt(52-32)=4 by Pythagoras. The area of ABC =1/2ACBM=1/264=12. We can see that the side lengths of DEF are greater than the side lengths of ABC by a factor of two. The area is therefore greater than ABC by a factor of 22=4. So the area of DEF=4*12=48