21. A general system of linear equations is

[tex]\[
\begin{array}{l}
a x + b y = e \\
c x + d y = f
\end{array}
\][/tex]

where [tex]\(a\)[/tex], [tex]\(b\)[/tex], [tex]\(c\)[/tex], [tex]\(d\)[/tex], [tex]\(e\)[/tex], and [tex]\(f\)[/tex] are constant values.

a) Use elimination to solve for [tex]\(x\)[/tex] and [tex]\(y\)[/tex] in terms of [tex]\(a\)[/tex], [tex]\(b\)[/tex], [tex]\(c\)[/tex], [tex]\(d\)[/tex], [tex]\(e\)[/tex], and [tex]\(f\)[/tex].

b) Are there any values that [tex]\(a\)[/tex], [tex]\(b\)[/tex], [tex]\(c\)[/tex], [tex]\(d\)[/tex], and [tex]\(f\)[/tex] cannot have?