Need explanations!

My test is coming up real soon!

Need help with these questions.

Sorry for the rudeness ... tests make me... nervous...

Describe all vectors v = [x,y] that are orthogonal to u = [a,b].

And I really really need help understanding "span" in linear algebra. How do you determine if a vector is the span of a matrix?

prob) determine if the vector b is in the span of the columns of the matrix A.

A =

[1 2]

[3 4]

b =

[5]

[6]

prob) show that R^2 =

span

([1] , [1])

([1] , [-1])