Now, when it comes to resonant frequency is relatively simple, since that is the frequency when impedance is lowest possible. Impedance here is Z=R+ j(ωL - 1/(ωC)) it means that lowest value of impedance will be when ωL=1/(ωC) or

ω=1/sqrt(LC) or f=1/(2πsqrt(LC)) and that's what's going on with frequency when we have maximal value of current, but how can i find frequencies (i believe there should be two frequencies where I=Imax/sqrt(2)) such that they satisfy the requirement of this example.