In 5x-5y=-20 doesnt x= -4 and y = 4?????

so if I needed to know x-y it would be -4-4 which is -8?? So why does it say in my book that x-y = -4 ???

5x - 5y = - 20 divide through by 5

x - y = -4

This is the equation of a line.......there are infinite values that make this true...for example

( x, y ) = ( 0, 4) = ( 4, 8) = (-4, 0 ) .....etc.