Problem
Explain why there does not exist a list of six polynomials that is linearly independent in $\mathcal{P}_4(\mathbb{F})$
Solution
The list $1,z,z^2,z^3,z^4$ has five linearly independent elements and spans $\mathcal{P}_4(\mathbb{F})$, therefore by theorem $2.23$ no list with more than 5 elements that spans the space can be linearly independent.