Use the distributive property:
color(magenta)a(color(red)x+color(blue)y)=color(magenta)a*color(red)x+color(magenta)a*color(blue)y
Here's this property applied to our expression:
color(white)=color(magenta)(2/5)(color(red)(5k)+color(blue)35)-8
=color(magenta)(2/5)*color(red)(5k)+color(magenta)(2/5)*color(blue)35-8
=color(magenta)(2/color(red)cancelcolor(black)5)*color(red)(color(black)cancelcolor(red)5k)+color(magenta)(2/5)*color(blue)35-8
=color(magenta)2color(red)k+color(magenta)(2/5)*color(blue)35-8
=color(magenta)2color(red)k+color(magenta)((2color(black)*color(blue)35)/5)-8
=color(magenta)2color(red)k+color(magenta)(color(purple)70/5)-8
=color(magenta)2color(red)k+color(purple)14-8
=color(magenta)2color(red)k+6
That's the expanded expression. Hope this helped!