# Different fonts in the same Style in a Plot

I have a plot, not important what it is and I have a linelegend as below:

PlotLegends ->
Placed[LineLegend[{Style["K=3", FontSize -> 23],
Style["K=5", FontSize -> 23], Style["K=7", FontSize -> 23],
Style["K=9", FontSize -> 23]},


Here all "K" s must be italic but not "=" and the following numbers. In the usual case all are roman. How can I deal with this problem?

Plot[{3 x, 5 x, 7 x, 9 x}, {x, -1, 1},

PlotLegends -> Placed[LineLegend[