QUESTION $3$

An ideal gas goes through the process shown below. From $\backslash ($ A $\backslash )$ to $\backslash ($ B $\backslash ),$ the process is adiabatic; from $\backslash ($ B $\backslash )$ to $\backslash ($ C $\backslash ),$ it is isobaric with $345$ kJ of energy entering the system by heat. From $\backslash ($ C $\backslash )$ to $\backslash ($ D $\backslash ),$ the process is isothermal; and from $\backslash ($ D $\backslash )$ to $\backslash ($ A $\backslash ),$ it is isobaric with $371$ kJ of energy leaving the system by heat. Determine the difference in internal energy $($in kJ $),\backslash (\backslash$mathrm$\{$E$\}\_\{\backslash$mathrm$\{$int$\},\backslash$mathrm$\{$B$\}\}-\backslash$mathrm$\{$E$\}\_\{\backslash$mathrm$\{$int$\},\}\backslash$mathrm$\{$A$\}\backslash ).$I calculate the answer $-18\mathrm{.}9,$can you tell me why the correct answer is $18\mathrm{.}9$