y=-5-x

y = -5 - x    (1)

x = 5 - x   →  2x = 5    →  -5  = -2x   (2)

Subbing (2) into (1), we have

y = -2x - x

y = -3x   →     y/-3   = x

And using (2)

y/-3 = 5 - y/-3

2y/-3  = 5

[-2/3]y = 5    multiply both sides by -3/2

y = -15/2

And x =  y/-3 =  (-15/2)/ (-3)  = 15/6  =  5/2

So....the solution is   (x, y)  = ( 5/2, -15/2)