Answer:

Where a,b,c,d represent constants.
For case given we have this:
And we need that
That's satisfied if 

Where 
So then a possible basis for this case would be:
![B= [x^3 -1, x^2 -1 , x-1]](https://tex.z-dn.net/?f=%20B%3D%20%5Bx%5E3%20-1%2C%20x%5E2%20-1%20%2C%20x-1%5D)
And we can check that the spans for the subspace is linearly independent.
Step-by-step explanation:
We want to find a basis for the subspace of all polynomials with degree
and we need to satisfy the condition that 
Our general expression for an element of the space described above is given by this equation:

Where a,b,c,d represent constants.
For case given we have this:
And we need that
That's satisfied if 
So we want a basis for the subsapce of all the polynomials on this form:

Where 
So then a possible basis for this case would be:
![B= [x^3 -1, x^2 -1 , x-1]](https://tex.z-dn.net/?f=%20B%3D%20%5Bx%5E3%20-1%2C%20x%5E2%20-1%20%2C%20x-1%5D)
And we can check that the spans for the subspace is linearly independent.