Help proving a subset is a subspace

[paulrb]

Homework Statement

Prove that the set of all 3-vectors orthogonal to [1, -1, 4] forms a subspace of R^3.

Homework Equations

Orthogonal means dot product is 0.

The Attempt at a Solution

I know the vectors in this subspace are of the form
[a,b,c] where a - b + 4c = 0.
However I don't know how to use this to show there is closure under vector addition and scalar multiplication.

 

[dx]

Let v_1 and v_2 be two vectors orthogonal to the given vector (call it a), i.e.

v_1 \cdot a = 0
v_2 \cdot a = 0

Now, using these, all you have to do is show that

(v_1 + v_2) \cdot a = 0

HINT: Use distributivity of the dot product.