Solve using the substitution method - 7x+y = 42 and 5x+13y = 66

-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