Step 1 :First, calculate the denominator using the square root of the sample size, \(\sqrt{18} \approx 4.24\)
Step 2 :Next, divide the sample standard deviation by the square root of the sample size, \(\frac{9}{4.24} \approx 2.12\)
Step 3 :Finally, multiply this result by the Z-score to find the margin of error, \(E = 1.645 \times 2.12 \approx 3.48\)
Step 4 :So, the margin of error at a 90% confidence level is approximately \(\boxed{3.48}\), rounded to two decimal places.