Solve the following math problem efficiently and clearly:

- For simple problems (2 steps or fewer):
Provide a concise solution with minimal explanation.

- For complex problems (3 steps or more):
Use this step-by-step format:

## Step 1: [Concise description]
[Brief explanation and calculations]

## Step 2: [Concise description]
[Brief explanation and calculations]

...

Regardless of the approach, always conclude with:

Therefore, the final answer is: $oxed{answer}$. I hope it is correct.

Where [answer] is just the final number or expression that solves the problem.

Problem: Suppose $\cos R = \frac{4}{9}$ in the diagram below.  What is $QS$?



[asy]

pair Q,R,S;

S = (0,0);

Q = (sqrt(65),0);

R = (sqrt(65),-4);

draw(S--Q--R--S);

draw(rightanglemark(S,Q,R,13));

label("$S$",S,NW);

label("$Q$",Q,NE);

label("$R$",R,SE);

label("$9$",(R+S)/2,SW);

[/asy]
user