we know that triangle APQ is an equilateral triangle because it has a 60 degrees angle.
a)which means that PQ=AP=AQ=2cm.
b)you can use congreunce of triangles PDC and CBQ. where you have 2 equal lines DP and QB
and 2 equal opposite angles in the rhombus.
then you got b)PC=QC
c) looking at triangle QBC:
then you get QC with the cosine rule.
then you use the cosine rule in triangle PQC