Simply substitute b = -2 into the expression of b^2 + 9b + 3:
All the b in the expression will become -2.
(-2)^2 + 9(-2) + 3 {The power of 2 means the number is multiplied twice} = (-2)(-2) + (-18) + 3 {Negative number times negative number will give you positive number} = (4) - 18 + 3 = 4 - 18 + 3 = 4 + 3 - 18 = 7 - 18 = -11