A linear equation is an algebraic equation that contains terms which can either be constants or variables. The variables can only be of the first power.
A linear equation in two variables is of the form ax + by + c = 0, where x and y are variables and a, b and c are real numbers. Additionally a and b are non zero.
Learnhive Lesson on Linear Equation in Two Variables
Learnhive Lesson on Simultaneous Linear Equation in Two Variables