Answer :
let's keep in mind that Profit = Revenue - Costs.
so each guitar costs originally "d" dollars online.
you buy the kits at half price, 50% of the original price, well, the original price is "d", half "d" is just 0.5d, and there's a shipping charge of $4.50. So the Cost to get one kit alone for you is 0.5d + 4.50.
After some work, you resell the kits online for 1.5d + 4.5, so since that's the selling price that is your Revenue for one kit.
[tex]\stackrel{\textit{\Large Profit}}{\stackrel{Revenue}{(1.5d~~ + ~~4.50)}~~ - ~~\stackrel{Costs}{(0.5d~~ + ~~4.50)}}\implies 1.5d+4.5-0.5d-4.5 \\\\\\ 1.5d-0.5d+4.5-4.5\implies 1d\implies d[/tex]