When we're talking about line segments, we just have to find the sum of the two parts of the line segment to find the value of the whole. In this case, we have x^2 + 25, and 10x. So, we add them together.
x^2 + 25 + 10x
= x^2 + 10x + 25
Since this can be immediately factored into (x + 5)^2, you have your answer.
