Using properties of parallelograms, prove
ΔAPD ≅ ΔCBQ then
∠APB ≅ ∠CQB by corresponding parts
AP || CQ by alternate exterior angles
AC bisects DB diagonals mutually bisect
Label intersection E
DE = BE definition of bisect
DE = DP + PE segments addition
BE = BQ + QE segments addition
DP + PE = BQ + QE substitution
DP = BQ given as trisection
PE = QE segment addition/subtraction