How can a line be linear and nonlinear?

Answer 1

Two possibilities:

The line is linear over some of its length and then goes non-linear (or the other way round:

Think of a mass, at the end of a string, being swung in a circle. Then the string is cut. The motion of the mass would have been circular (lon-linear) until the instant the string was cut and then linear, as it flies off into a tangent.

Or

The line is linear from one perspective but not from another:

Think of the trajectory of a ball that is thrown up at an angle to the horizon. If seen from above, the ball travels in a straight line (linear) but if seen from the side it follows a parabola (non-linear).

Hope that helps.