First, i am not sure what it means "it's internal resistance is known (R=20Ω)". Does it means that ωL=R=20Ω, or i can think of it as a resistor added is series with inductor and capacitor?

Then, i know that Imax=0.707Ieff and i know that:

i(t)=Imax*cos(ωt+φ) and ω=2πf and when i have frequencies f1=580kHz and f2=612kHz i(t)=Imax*0.707 but i don't know how this can help me.