Linear equations

Expression: A collection of numbers, variables, and signs, such as $3, 3x+4, 5 x^2 + 2, \sqrt{x-3},$etc

Equations: A mathematical statement with an equal sign, such as $y = 2, y = 3x-2, y = x, x = 3,$etc

Linear

Equation: Ax + By = C (A, B & C are constants; x & y are variables)
All linear equations are functions except a vertical line such as x = 3.