$\mathrm{7,1}\text{ cm}=71\text{ mm}7\{,\}1~\backslash text\{cm\}=71~\backslash text\{mm\}$

$71\text{ mm}-6\text{ mm}=65\text{ mm}71~\backslash text\{mm\}-6~\backslash text\{mm\}=65~\backslash text\{mm\}$

Das Brett ist $65\text{ mm}65~\backslash text\{mm\}$ oder $\mathrm{6,5}\text{ cm}6\{,\}5~\backslash text\{cm\}$ dick.