The solution is k=- 1 and b = 120.

The expression for finding a linear function is y =-x+ 120.

2)W=(x-60)？ 6? 1(-x+ 120)

=-x2+ 180x-7200

=-(x-90)2+900

The opening of the parabola is downward,

When x < 90, w increases with the increase of x,

And 60≤x≤87,

When x=87, w =-(87-90) 2+900 = 89 1.

When the sales unit price is set to 87 yuan, the mall can get the maximum profit, which is 89 1 yuan.

3) From w = 500,500 =-x2+180x-W=500,

After sorting, x2- 180x+7700=0,

Solution, x 1=70, X2 = 1 10. (7 points)

As can be seen from the image, in order to make the profit of this mall not lower than that of 500 yuan, the sales unit price should be between 70 yuan and 1 10 yuan.

60 yuan ≤x≤87 yuan/set, so the range of sales unit price X is 70 yuan ≤x≤87 yuan/set.