Flip flop 2 time delay will be twice of delay in flipflop1 i.e 2*40=80ns.
Flip flop1 and Flipflop3 are synchronous so their time delays will be 40ns.
The output will be 80ns from flipflop2 + 20 ns from AND gate +40 ns from Flipflop3 = 140 ns total.
So frequency = 1/140 nsec= 7.14 MHz