A fully-clamped graphene/poly(methyl methacrylate) (PMMA) open cavity resonator with fundamental frequency within the lower ultrasonic frequency range (less than 100 kHz) has been fabricated and the dynamic behavior of the graphene/PMMA resonator has been observed and characterized in atmospheric pressure and at room temperature. The challenge in achieving the fundamental frequency less than 100 kHz in resonators made by graphene-based material is the handling of the large size of the thin membrane and the need to modify the fabrication method to prevent collapse of the membrane. In this work, graphene/PMMA membrane consisting of 6-layer graphene and 450 nm poly(methyl methacrylate) (PMMA) has been wet transferred over a square cavity with the side length of 0.5 mm. The resonant frequency has been measured to be around 75 kHz by mechanical actuation and 79 kHz by electro-thermal actuation. The (0,1) mode has been detected to dominate the resonance. The simulated dynamic behavior of the graphene/PMMA membrane over the open cavity has been found to be in agreement with experimental measurements. The strain in the graphene/PMMA membrane has been estimated to be around 0.2 %.