Can someone help me with this question please.

Aportion of a line bounded by two points is defined as

What is the missing constant term in the perfect square that starts with x^2 -20x

Will mark brainliest for correct answer! i have been on this problem for an hour and can't find the answer. i'm begging here! incase you cant read the instructions in the picture because of too much or too little lighting: note: enter your answer and show all the steps that you use to solve this problem in the space provided. write a list of steps that are needed to find the measure of b. provide all steps as clear as possible, !