If x2 - 2xy = 84 and x - y = -10, then the value of y is:

x² − 2xy = 84

x = y - 10

According to question,

x² − 2xy = 84 ------------(a)

And x = y - 10

now, put x = y - 10 in (a)

(y - 10)² - 2y(y - 10) = 84

= y² - 20y + 100 - 2y² + 20y = 84

= - y² = 84 - 100

= y² = 16 

= y = 4