Hey there.
While this may seem confusing at first, it's much more simple than it will ever appear. Given we have a total length, 4x + 1, and a segment length, 3x - 4, we just need to subtract the given segment from our total to solve for our other segment.
(4x + 1) - (3x - 4); we cannot subtract numbers from variables, so only subtract like-terms.
4x - 3x = 1x (or just 'x')
1 - (-4) = 5;
Put our difference of terms together;
1x + 5;
Our correct answer is "C.) x + 5"
I hope this helps!