What is the zeros of 3x^2+9x+29

None. This function does not cross the x axis. The "solutions" or "zeros" are imaginary.

How do you know

Because the discriminate is a negative number, and the square root of a minus number does not lend itself to anything that will cross the x axis.

a = 3

b = 9

c = 29

D = b^2 - 4ac

D = 9^2 - 4*3*29

D = 81 - 348

D = - 268

To further convince you, I have included the graph for this.