Hey! Let's solve this shall we ?
Since the revenue from selling x necklaces is r(x) = 10x and the cost of buying x necklaces is c(x) = 4x + 15, the profit function p(x) = r(x) - c(x) yields,
p(x) = r(x) - c(x)
= (10x) - (4x+15)..........Distribute the negative inside to change the signs
= 10x - 4x - 15.........Then add like terms!
p(x) = 6x - 15
I hope this helped!