please help me to prove this exercise in modern algebra

please help me to prove this exercise in modern algebra

Suppose $W$ be a free R-module with basis $X=\{x_1,...,x_n\}$ . If $\phi:
W \rightarrow W$ is an R-module endomorphism with matrix $A$ relative to
$X$ , then the determinant of the endomorphism $\phi$ is defined to be the
determinant $|A| \in R$ and is denoted $|\phi |$. How to show $|\phi |$ is
independent of the choice of $X$?
thanks in advanced..