Represent the walls of the canyon as A and B and the man by M between A and B, then AB=AM+MB.
The echoes are caused by reflection, and the sound has to travel from M to A and back again, so the distance is 2AM. Similarly, between M and B the distance is 2MB.
So 2AM=340×1.4 and 2MB=340×2.2, therefore AB=340×1.4/2+340×2.2/2=340(0.7+1.1)=340×1.8=612m.