Not sure but It should be -4/1
X=20
If they're parallel the other number in the same spot as 5x+15 equals that, so 5x+15=115 because if they are parallel they're the same if same spot and same line going through them.
115-15 = 100
5x=100
Divide by 5 both sides
X=20
If x=20 then these lines would be parallel
Two lines that are parallel have the same slope. Thus, find the slope of the line (J) thru (4,2) and (5,1), and that will be the answer to your question.
2-1 1
m = --------- = ------ = -1 (answer)
4-5 -1