$14.)$ Prove the following using fundamental definitions.

a$)$ For sets $A,B,$ and $C,$ it follows that $(AC)-B=(A-B)C.$

b$)$ For sets $P,Q,$ and $R,$ it holds that $(PQ)-$RsubeP$(Q-R).$

C$)$ For sets $P,Q,R,$ and $T$ such that PsubeR, it holds that $P(QT)sube(PQ)(RT).$