According to question conditions;
Number of MP3 players = x
While combined numbers of both MP3 and DVD players = 113
So number of DVD players = 113 – x
Profit on single MP3 player = $35
Profit on single DVD player = $18
Total profit on all MP3 player = ($35)(x) -------- (1)
Total profit on all DVD player = ($18)(113 – x) ---------- (2)
Combined profit from MP3 and DVD players are obtained by adding both equations (1) and (2), i.e.
= 35x + 18(113 – x) = 35x + 2034 – 18x = $(17x – 2034)