If a function is linear, that means the degree on the x variable is 1 when solved for y. This means that the exponent on the x is 1. For example:
y²=x
y=√x
y=x^1/2
This is not a linear function because the power on the x function is not 1
x+y=3
y=-x+3
This is a linear function because the exponent on the x is 1.