Suppose you know x+2y=-1$ and 3y-x=-19+x. What is xy?
x + 2y = -1 → x = -1 -2y (1)
3y - x = -19 + x (2)
Sub (1) into (2)
3y - (-1 -2y) = -19 + (-1 -2y)
3y + 1 + 2y = -19 -1 -2y
5y + 1 = -20 - 2y
7y = -21
y = -3 and x = (-1 -2(-3)) = 5
xy = 5(-3) = -15
