expanding and simplifying a function to determine if it is linear or quadratic

I haven't done this in a while so I can't remember how to do everything, just when I thought I had it figured out, my answer did not match with the correct one. Here's the question:

6.State the degree of each function, and identify which functions are linear and which are quadratic.

b) m(x) = -x^2 + (x +3)^2

Here is my answer:

m (x) = -x^2 + (x + 3)(x + 3)

= -x^2 + x(x + 3) + 3(x + 3)

= -x^2 + x^2 + 3x + 3x +9

= 6x + 9

The degree is 1 because the highest exponent of x is 1. The function is linear

Apparently the answer is : the degree is 2 and the function is quadratic. I know I must have not done at least one thing right here... hopefully someone can tell me what that is.

Thanks in advance

PS.the latex thing doesn't seem to want to work on my computer sorry