For these problems, determine the number of zeros using the discriminant, then solve using the quadratic formula. Leave your answer in simplified radical form, if necessary.

1. y = x^2 + 5x - 15

2. y = 2x^2 + 16x + 32

3. y = x^2 + 18x + 4

4. y = x^2 - 24x - 6