The first we need to notice is that the length of the black line is the addition of every segment in which the black line is divided.
So, the total length of the black line is given by:
Therefore, the length of the black line is:
Finally, we know that the length of the black segment is equal to that of the green segment, in equation form this looks like:
And the length of the green segment is equal to 48
And that's the equation we are looking for
We can go further and solve it