The person completing the question will need to spilt the shape up so that they can find which rectangle is equal to 18cm squared. Then, they will have to work out what the area of the remaining rectangle is, which belongs to the other son.
They will need to find the missing length of A and B/ or how much of 5cm is used for that rectangle
A= 6cm-2cm= 4cm
B= 5cm-3cm= 2cm
5cm (C)= 5cm=3cm=2cm
Using either A x B or A x C= 8cm squared area of the field