The slope of the tangent to a curve at any point is given by the derivative of the function at that point. The given function is \( y = x^2 \).
Step 1: Find the derivative of the function.
The derivative of \( y = x^2 \) with respect to \( x \) is:
\[
\frac{dy}{dx} = 2x.
\]
Step 2: Find the slope at \( x = 3 \).
Substitute \( x = 3 \) into the derivative:
\[
\frac{dy}{dx} \bigg|_{x=3} = 2(3) = 6.
\]
Thus, the slope of the tangent at \( (3, 9) \) is 6.
Final Answer:
\[
\boxed{\text{(B) 6}}.
\]