I dunno how to solve this!!!WAVES?

Question

Hello.Here is a problem i can't solve, so i am asking your help!

Two points K and L of an elastic medium,which is doing a harmonic wave with a frequency f=90 Hz, have phases at the moment t1, phase(K)=15π/4 rad and phase(L)=3π/2 rad.How much time needs the wave to cover the distance of those 2 points???

Answer 1

The speed of the wave is Δx/Δt, where
Δx = (K-L)
Δt = (Θ(K)-Θ(L))/ω, where ω = 2pi*f
I can give you Δt. It's 0.0125 s. If K and L are 1 m apart, v = 80 m/s.