(a)[1]
Determine $\angle DCB$.
(b(i))[1]
Construct the locus of points, inside the quadrilateral, that are $8$ cm from $B$.
(b(ii))[1]
Construct the locus of points, inside the quadrilateral, that are $5$ cm from $AB$.
(b(iii))[1]
The two loci intersect at $P$. Mark the point $P$ on the diagram and label it, then measure $PD$.