Respuesta :

pead
-7x+y=42
y=42+7x 

5x+13y=66
Substitute for y (meaning put 42+7x into the equation above instead of y because they are equal - see second line of my answer above)

5x+13[42+7x]  = 66
5x +546 = 66
5x=66-546
5x=-480
x=-480/5
x=-96