linear or nonlinear??y=2x−9y=−10.2y=3x^2+73x + 5y = 15

Linear equations are first degree equations, which means that x is only raised to the first power.  Nonlinear equations are second-degree equations, third-degree equations, fourth degree, etc., that have x raised to the second, third, etc., power.  So in order to tell if it is linear or not, you just have to see if x is raised to a power or not.

1. Linear, since x is to the first power
2. Linear, since it will just form a line since y will always equal the same value.
3. Nonlinear, since x is raised to the second power, and will, therefore, create a parabola.
4. Linear, since x is once again raised to the third power.