Question

Find common solution for x+y=8 x=-5y

Answer 1

x = -5y

Substitute the value of x in first equation, 
x + y = 8
(-5y) + y = 8
-4y = 8
y = -8/4
y = -2

Now, substitute the value of y in 2nd equation, 
x = -5(-2)
x = 10

In short, Your Answer would be; (10, -2)
Hope this helps!