Are you having trouble understanding in general finding the limits of y as x approaches something or are the steps in that example confusing you? I find the +/- and +/+ really confusing too and wouldn't recommend thinking about limits like that. But it's just trying to say that as x approaches 3 from the negative side, take a number like 2.9 for example, when you sub it into x^2/(x^2-9) the numerator will be positive and the denominator will be negative. A positive number divided by a negative number is a negative number, thus y will approach negative infinity.