Any equation of the form y = mx+ b is a linear function

The variable x must be either degree zero or degree 1 AND the variable y must be 1st degree in order to be a linear function.

Examples:

y = 2x - 3 (both x and y are 1st degree)

4x + 5y = 20 (both x and y are first degree)

2x - 4y = 7 + 3x (all variables are 1st degree)

Any equation which is not of the above mentioned form is a nonlinear function

If variable y is 1st degree but the variable x has a degree other than 0 or 1, it will be a non-linear function of x.

Examples:

y = x^2 + 25(x is not first degree)

y = 5x + 2 - x^3 (x is 3rd degree)

Skills to recall:

What is does degree in math is

https://www.mathsisfun.com/algebra/degree-expression.html

What is a variable

https://www.mathsisfun.com/definitions/variable.html

What is an equation

https://www.eduplace.com/math/mathsteps/7/d/index.html