So k+2=3k gives k= 1.

So a=( 1, 1) so a×b= 1×2+ 1×2=4.

Solution: It is known that the direction of vector A is opposite to that of vector B, and b=(2, 1).

Let the vector a=(-2k, -k)

And | a | = 2 √ 5.

So 5k? =(2√5)?

K = 2, A and B are in opposite directions.

So the vector a=(-4, -2)a+bx=( 1+x, 2) should be parallel to c=(3, 4).

therefore

( 1+x)/3=2/4= 1/2

So 2( 1+x)=3.

2+2x=3

2x= 1

x= 1/2

Don't forget the arrow! ! ! !