The current price is: original price+10x = 50+ 10x. ...

The total number of pieces sold is: original total sales-10x=300- 10x.

Cost price: cost * total number of pieces =30*(300- 10x)

Now profit = current price * total number of pieces-cost, so:

(50+ 10x)(300- 10x)-30(300- 10x)= 8700

15000-500 x+3000 x- 100 x？ -9000+300x=8700

2800x+6000- 100x？ =8700

100x？ -2800x+2700=0

x？ -28+27=0

(x- 14)2- 169 = 0// Here is the square of x- 14- 169 = 0.

(x- 14)2 = 13 2// Here is the square of x- 14 = 13.

So x- 14 = 13.

When x- 14= 13, X=27.

When x- 14=- 13, X= 1.

With the original test, when x=27 or x= 1, the monthly profit can be 8700 yuan.

When x=27, bring in ①.

30+ 10*27=300 (yuan)

When x= 1, it is brought into formula ①.

30+ 10* 1=40 (yuan)

Therefore, when each underwear is priced in 40 yuan or 300 yuan, the monthly profit can reach 8700 yuan.

Sincerely, Liuzhou Oriental Voice car audio refit shop, hope to adopt.