A shark's depth in meters relative to sea level over time in minutes, x, is modeled as a parabola. At time 1 minute the shark is swimming upwards and is at a depth of 7 meters. The shark continues to swim toward the surface for another minute and then turns and begins to swim downwards and returns to a depth of 7 meters at time 3 minutes. What is the axis of symmetry of the graph of the parabola?