$2\mathrm{.}6.$ One mole of gas in a closed system undergoes a four$-$step thermodynamic cycle. Use

the data given in the following table to determine numerical values for the missing

quantities indicated by question marks. $[$ Please answer that: Since we do not know the first step and internal energy is a state function, why and how the last step is calculated as $0.$How can we find work and heat by adding them up when we don't know the intermediate steps?