Linear equation
Every linear equation has a variable $x$ that does not occur in any higher power (for example, $x^2$ or $x^3$). A linear equation can have one, two or more unknowns. The general form for a linear equation with one variable is:
$ax+b=0$
$a$ and $b...$ real numbers
$x ...$ unknown variable
i
Tip
Most often, the variable is denoted by $x$ in linear equations. Note, however, that even equations with other letters can be linear equations.
Examples

Linear equations (with one variable)
$3x+15=0$
$5x+3=0$
$7b=14$ 
No linear equations
x occurs at a higher power (e.g. $x^2$ or $x^5$)
$x^2+4=29$
$5x^5+12y+18=0$