Set up a proportion.
The total side of the “top” line is 8 + 10 = 18. The total side of the “bottom” line is 45. This makes the ratio 18/45.
Now we are trying to find the “larger” bottom segment, let it be x. Because the lines are parallel, the ratio of the “larger” top and bottom segments is equal to the ratio of the whole top and bottom lines.
So 18/45 = 10/x
Reduce 2/5 = 10/x
Cross multiply 2x = 50
Solve x = 25