Consider two unique parallel lines. what aspects of these two lines are the same? what aspects of these two lines would have to be different? explain your reasoning.

Parallel lines must have the same slope.  However for them to be UNIQUE lines, ie different lines, they must have a different y-intercept.

So if we say generally that a line is y=mx+b where m is the slope and b is the y-intercept then these two unique parallel lines would be:

y1=mx+h and y2=mx+k

Where m is the same for both and each have unique constants h and k where they cross the y-axis