How do you represent #"2-methyl, 3-ethylnonane"#?

Structural formula:(Hydrogen is attached to complete Every carbon's valency {but I've not shown here as it is understood, so don't get confused)

#color(white)(a.aaaaaaaaaaa)"CH"_2-"CH"_3#
#color(white)(aaaaaaaaaaaaa)|#
#color(white)(aa.aa)"C"-"C"-"C"-"C"-"C"-"C"-"C"-"C"-"C"#
#color(white)(aaaaaaaaa)|#
#color(white)(aaaaaaaa.)"CH"_3#

Condensed Structural formula:

#"CH"_3-"CH"("CH"_3)-"CH"("CH"_2-"CH"_3)-"CH"_2-"CH"_2-"CH"_2-"CH"_2-"CH"_2-"CH"_3#

Draw the alkane parent, the nonane:

#"H"_3"CCH"_2"CH"_2"CH"_2"CH"_2"CH"_2"CH"_2"CH"_2"CH"_3#

Single out the carbons on the chain that require substitution: #"C"_2# and #"C"_3#:

#"H"_3"C"color(red)("C")"H"_2color(red)("C")"H"_2"CH"_2"CH"_2"CH"_2"CH"_2"CH"_2"CH"_3#

Substitute these positions with #"methyl"# and #"ethyl"# groups:

#"H"_3"C"color(red)("C")"H"("CH"_3)color(red)("C")"H"("CH"_2"CH"_3)"CH"_2"CH"_2"CH"_2"CH"_2"CH"_2"CH"_3#

Of course, you could condense this a bit; it is important to get the condensed formula right, i.e. #"C"_12"H"_26#, i.e. this follows the general formula, #C_nH_("2n+2")#, a saturated alkane as required.