  Given a linear equation $\mathcal{L}$, a set $A \subseteq [n]$ is
$\mathcal{L}$-free if $A$ does not contain any `non-trivial' solutions to
$\mathcal{L}$. In this paper we consider the following three general questions:
  (i) What is the size of the largest $\mathcal{L}$-free subset of $[n]$?
  (ii) How many $\mathcal{L}$-free subsets of $[n]$ are there?
  (iii) How many maximal $\mathcal{L}$-free subsets of $[n]$ are there?
  We completely resolve (i) in the case when $\mathcal{L}$ is the equation
$px+qy=z$ for fixed $p,q\in \mathbb N$ where $p\geq 2$. Further, up to a
multiplicative constant, we answer (ii) for a wide class of such equations
$\mathcal{L}$, thereby refining a special case of a result of Green. We also
give various bounds on the number of maximal $\mathcal{L}$-free subsets of
$[n]$ for three-variable homogeneous linear equations $\mathcal{L}$. For this,
we make use of container and removal lemmas of Green.
