Question 667831
x+y =5  and xy=4

y = 5-x and plug into xy=4 to get:

x(5-x) = 4

5x - x^2 = 4

x^2 -5x +4 = 0

(x-1)(x-4) = 0 

x = 1 and x = 4

When x =1,  y = 4

When x =4, y =1

So in case 1:  x-y = 1-4 = -3

Case 2:  x-y = 4-1 = 3