Answer :
Cost of your guitar = $ 600
Let your friend's guitar's cost be = n
According to the question =
Your guitar costs $ 40 more than twice the cost of your friend's guitar which is n ,but your guitar's cost is $ 600.
So ,the linear equation for this will be =
= 2n+$40 = $600
= 2n = $600-$40
= 2n = $ 560
= n = $ 560/2
= n = $ 280
Thus ,the cost of your friend's guitar is $ 280.