Question

Are linear equations only in one variable?

Answer 1

No linear equations are also in two variables,three variables,four and so on

linear equations in two varibales-In generalized form it can be written as ax+by = c where a, b, c are constants and x, y are variables.
In three variables
ax+by+cz=m
where a,b,c and m are constants and x and y are variables