Simple. Since you approach from the left of -3, try to think of a number right next to -3, like -3.1
Then, you see that If you solve for that number, your value in the denominator will be a very small negative number.
Then, in the numerator the number will be a negative number.
So, since you divide a number by a very small number that is extremely close to zero, you get infinity, and since both are negative, they cancel and you get possitive infinity.