Latex ! Undefined control sequence

A Guide To Latex (H Kopka, P Daly) 4Ed
P402 Appendix C. Error Messages

! Undefined control sequence.
l.3 The last words appear in \txetbf
{bold face}.
?

! Undefined control sequence, meaning that an unknown command name (control sequence) was the cause of the error. Next comes a pair of text lines, the first of which is prefixed with 1.3, meaning that the error occurred in 'line 3' of the input text. The error itself was encountered at the last symbol printed in the upper line. The lower line shows the continuation of the input line being processed when the error was found, here the words {bold face}. Before continuing, TEX waits for a reaction from the user, as indicated by the question mark in the last line of the message.

For my case:

! Undefined control sequence.

l.9 A random vector \underscore

                               {x}:

should be changed to \underline{x}

fixed