The answer would be a positive 15.1 or +15.1 because the positive number is greater than the negative which would make your answer positive, thats why you got it wrong :)
To solve you would use the equation y=mx=b . y is how far line goes up, x is how for along , m is the slope and b is where the line crosses the yaxis. slope: -1 x-intercept ; -4 y-intercept : -4