\documentclass[a4paper,12pt]{article}
\newcommand\ds{\displaystyle}
\begin{document}

\parindent=0pt

QUESTION

State a condition on the matrix {\bf A} for the general system of
$n$ equations in $n$ unknowns given by {\bf AX}={\bf b} to have a
unique solution.

\bigskip

ANSWER

{\bf AX}={\bf b} has a unique solution if $\det\mathbf{A}\neq0$.




\end{document}