As long as the slopes of straight line AB and straight line BC are equal.

∴(3-a)/(- 1)=(3-b)/(-2)

The solution is 2a-b=3.

2、D

The original formula =(AC+CD)-(AB+BD)=AD-AD=0.

3、c=2a-b

Let c=xa+yb.

Through the vector coordinate formula

4=x-2y

1=2x+3y

X=2 y=- 1。

4、D

If three vectors can be connected end to end to form a triangle

These three vectors add up to 0.

You can draw a picture yourself. It's very simple.

∴4a+(3b-2a)+c=0

Substitute a and b to get

c=(4，-6)