  Multiple-ribbon flares are usually complex in their magnetic topologies and
eruption mechanisms. In this paper, we investigate an X2.1 flare
(SOL2015-03-11T16:22) that occurred in active region 12297 near the center of
the solar disk by both potential and nonlinear force-free field models
extrapolated with the data observed by the Helioseismic and Magnetic Imager
(HMI) on board Solar Dynamics Observatory (SDO). We calculate the
three-dimensional squashing degree distribution. The results reveal that there
are two flux ropes in this active region, covered by a large scale hyperbolic
flux tube (HFT), which is the intersection of quasi-separatrix layers with a
null point embedded in it. When the background magnetic field diminishes due to
the separation of the northwest dipole and the flux cancellation, the central
flux rope rises up forming the two brightest central ribbons. It then squeezes
the upper lying HFT structure to generate further brightenings. This very
energetic flare with a complex shape is accompanied by a coronal mass ejection
(CME). We adopt the simplified line-tied force-balance equation of the current
ring model and assign the observed value of the decay index to the equation to
simulate the acceleration profile of the CME in the early stage. It is found
that the path with an inclination of $45^\circ$ from radial best fits the
profile of the actual acceleration.
