To find the missing length \( l \) of a rectangular prism when the height \( h \) and width \( w \) are both given, and the surface area \( SA \) is known, we start with the formula for the surface area of a rectangular prism:
\[ SA = 2(hw + hl + wl) \]
where:
- \( h \) is the height,
- \( w \) is the width,
- \( l \) is the length.
In this problem, we know:
- \( h = 9 \) cm,
- \( w = 9 \) cm,
- \( SA = 432 \) cm\(^2\).
Substituting the known values into the surface area formula:
\[ 432 = 2(9 \cdot 9 + 9 \cdot l + 9 \cdot l) \]
This simplifies to:
\[ 432 = 2(81 + 9l + 9l) \] \[ 432 = 2(81 + 18l) \] \[ 432 = 162 + 36l \]
Now, isolate \( l \):
\[ 432 - 162 = 36l \] \[ 270 = 36l \] \[ l = \frac{270}{36} \] \[ l = 7.5 \]
Thus, the missing length \( l \) of the rectangular prism, rounded to the nearest tenth, is:
\[ \boxed{7.5} \text{ cm} \]