A phase diagram is a temperature vs pressure plot from which the phase of a compound can be deduced under the given temperature and pressure conditions. The 3 equilibrium curves: Solid-Gas, Liquid-Gas and Solid-Liquid separate the compound into the three states of matter. The 3 states can however coexist at a unique point referred to as the triple point.
Based on the phase diagram for CO2 (which is readily available online), at -70 C and 5.5 atm, CO2 exist as a solid.