Geometric multiplicity of the largest eigen value

Question

let $$A= \begin{pmatrix} a & 2f & 0 \\ 2f & b & 3f \\ 0 & 3f & c \\ \end{pmatrix} $$ where $a,b,c,f$ are real numbers and $f\neq 0$. The geometric multiplicity of the largest eigenvalue of $A$ equals ?