Could I get help with this code? I keep getting error messages;

Unrecognized function or variable $\text{'}$k$\_$metal'.

Error in Mashtuntank$>$calculate$\_$resistance $($line $20)$

R$\_$top $=$ dx$\_$ins $/($k$\_$metal $*$ A$\_$top$)+1/($htop $*$ A$\_$top$)$;

Error in Mashtuntank $($line $33)$

R$\_$total $=$ calculate$\_$resistance$($dx$\_$ins, r$1,$ H$)$;