Start with the perimeter ABCF=26=2×7+2AF, from which 2AF=26-14=12, so AF=BC=6. All linear measurements are cms.
Split the trapezium CDEF into a rectangle CNEF and a triangle NDE by inserting the perpendicular EN from E onto BD. Length of AE=AF+EF=6+4=10, and BN=AE=10, and ND=BD-BN=14-10=4.
The area of CDEF=area(CNEF)+area(NDE)=4×7+½(7×4)=28+14=42 sq cm.