a. Lines are taken to lines, and line segments to line segments of the same length.

Since points are only locations, there's not much we can do with them transformation-wise. That being the case, the next logical step is moving on to lines and line segments.

The important distinction here is that line segments have defined length and lines don't. When thinking about lines, students only need to worry about slope and y-intercept (just like in a linear equation), while line segments have an additional element of length that students have to consider.

If we rotate a line, it would certainly change position. Depending on its initial position and how much we rotate it (or the number of degrees we rotate it by), it might go from horizontal to slanted, or from slanted to vertical, and so on. Reflection may or may not have the same effect on the line, but translation would change only the position (y-intercept) and not the slope.

Students should know that when we transform line segments in all these same ways, we'll end up with line segments with the same lengths. While the slopes and positions of the resulting line segments might be different from the original line segments, their lengths will always stay the same.

Have students play around with different transformations to see how they affect lines and line segments. Mirrors, paper folding, computer programs, or (old school) grid paper and string could be excellent tools to solidify students' understanding of the transformations and their effects.