Can somebody me. i don't understand.

So the first thing you need to do is convert the wavelength from centimeters to meters. This is quite easy, all you do is divide by 100.

Then I'm assuming the equation they're talking about is c = f λ. 

c = the speed of the wave
f = the frequency of the wave
λ = the wavelength

So all you need to do is multiply the frequency they gave you by the wavelength you calculated.

TOP ROW
f = 10 hz
λ = .1 m
c = 10×.1
c = 1 m/s

2ND ROW
f = 2.5 hz
λ = 10÷100
λ = .1 m
c = 2.5×.1
c = .25 m/s

3RD ROW
f = 30 hz
λ = 2÷100
λ = .02
c = 30×.02
c = .6 m/s

BOTTOM ROW
f = 50 hz
λ = 1÷100
λ = .01
c = 50×.01
c = .5 m/s