250=50k+b

100=200k+b

Solution:

k=- 1

b=300

The functional relationship between y and x is y =-x+300;

(2)∵y =-x+300；

When x= 120, y = 180.

The purchase price of brand A is RMB A, and the purchase price of brand B is RMB 2a.

120a+ 180×2a=7200，

Solution: a= 15,

B brand unit price 30 yuan.

Answer: The unit purchase price of two brands of stationery boxes is 15 yuan, 30 yuan;

(3) If brand A buys M, then brand B buys (-m+300).

15m+30(-m+300)≤6300

4m+9(-m+300)≥ 1795

Solution: 180≤m≤ 18 1,

∵m is an integer,

∴m= 180, 18 1.

* * There are two purchase schemes:

Scheme 1: A brand procurement 180, b brand procurement120;

Scheme 2: A brand purchase 18 1, B brand purchase119;

The profit from selling all the two brands of pencil cases is W yuan.

W=4m+9(-m+300)=-5m+2700。

∫k =-5 < 0，

∴W decreases with the increase of m,

∴m= 180, w max = 1800 yuan.

I hope the landlord will adopt it. Thank you.