Answer
$dim[Ker(T)]=2$
Work Step by Step
Since $T:P_5(R) \rightarrow M_2(R)$, we can obtain:
$Rng(T)=M_2(R)\\
\rightarrow dim[Rng(T)]=dim[M_2(R)]=4$
According to Rank-Nullity Theorem:
$dim[Ker(T)]+dim[Rng(T)]=dim[P_5(R)] \\
dim[Ker(T)]+4=6\\
dim[Ker(T)]=2$