According to Dalton's law of partial pressures, the total pressure is equivalent to the sum of the pressures of the individual gases (if they were present alone).
Therefore, the of $"N"_2$ is simply
$overbrace(450color(white)(l)"kPa")^(P_"total")$ $- overbrace(210color(white)(l)"kPa")^(P_("H"_2))$ $- overbrace(125color(white)(l)"kPa")^(P_(""O"_2))$ $= color(red)(115$ $color(red)("kPa"$