Which expresses all possible lengths of segment ab?

The solution is (2, -4).

We can solve this with the elimination method. Multiply the first equation by -2 and then add the two equations.

-20x + -12y = 8
20x + 10y = 0      Add the two equations

-2y = 8
y = -4

Now, just input -4 for y into either equation, you will get 2 for x.