Question

how to make graph of x+y=100

Answer 1

It's simple 

first substitute value of x and remove value of y 
do this 3 more times by substitutiung diffrent value of x and plot on graph

Answer 2

If x =0 then y= 100 ( 1+y= 100 , y =100)
If x =10 then y =90 (10+y=100, y= 100-10= 90)
If x =50 then y=50 (50 +y=100 ,y =100-50 = 50)
Make the graph accordingly..
I hope this helped youu