what is the difference between a linear interval and a nonlinear on a graph? how would they look?

In the case of linear equations, the graph will always be a line. In contrast, a nonlinear equation may look like a parabola if it is of degree 2, a curvy x-shape if it is of degree 3, or any curvy variation thereof. While linear equations are always straight, nonlinear equations often feature curves.

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It says in the questions that it is 6.5?