i want to GUESS. a heavy guess. that you rule out x+1 and you're left with x-3 over 3, or vise versa.
that would be wrong. you cannot rule out the x + 1 from the 3x + 1. if it were 3(x + 1) you could. but that is not the case. just multiply the numerators to get the new numerator, and the denominators to get the new denominator