3x+4y=12 is Linear.
y-1/2x+6 is Linear.
y=4x^2 is nonlinear.
y+2=|4| is linear.
y+6=4x is linear.
In order to tell whether an equation is linear or not, you have to first see if there is any variable^2 or more than ^1. If that is the case, it isn't linear. For example the equation x^2=4 or y^5=100 isn't linear. It also isn't linear if the equation is divided by X or Y.For example the equation 1/x=1 isn't linear or 1/y^2=5 isn't linear.
A question is also linear if you can write it in the form y=mx+b , 'm' meaning the slope of the equation and 'b' meaning the y-intercept of the equation.