\(Ax+By+Cz=D\)

\(\text{where } A,B,C,D \in R\)

The equations of a linear system with 3 variables are of the form:

\(A=m\)

\(B=-1\)

\(C=0\)

\(D=-n\)

A,B,C,D can have the values:

\(mx-y=-n\)

In this particular case, the equation is written:

\(mx-y+y+n=-n+y+n\)

\(y=mx+n\)

We add y+n on both sides and we have:

Thus it is possible to have equations in the form \(y=mx+n\) in a linear system with 3 variables which means the statement doesnt make sense.