Basic resistor question for parallel LED circuit
No, your calculation is not right.

Since your 9V source is a voltage source, and all LEDs are in parallel, the number of LEDs can't have any influence on the resistor. One LED can't "know" of its neighbors!

So, it's simply \$\frac{(9-3.4)\,\text{V}}{20\,\text{mA}}=280\,\text{Ω}\$.