How would you determine if the equation is a linear function

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