Did you see that formula at the bottom of the question ? The one that says. V = L x f ?
That's where you can stick the numbers that the question also gives you.
L = 3.4 m
f = 100 Hz
Puttum in the formula and then it says
V = (3.4 m) (100 Hz)
The question wants to know what ' v ' is.
Do that one multiplication and you'll have it.
It's 340 m/s.
Surprise surprise ! It's the speed of sound in air.