In synchronous circuits, we have, $T_{propagation-delay} = T_{flipflops} + T_{combinational}$ and In asynchronous circuits , We have : $T_{propagation-delay} = n*T_{flipflops} + T_{combinational}$
But in the question above, two FF's are given same clock (so these will respond to clock at same time) and the remaining Flip Flop is given the output of other Flip Flop ( so it will wait for output of first flip flop).
Here, $T_{propagation-delay} = 2*T_{flipflops} + T_{combinational}$ and $T_{clock}$ $\geq$ $T_{propagtion-delay}$.
$\frac{1}{freq_{clock}} \geq 2*30ns + 10ns$
$freq_{clock} \leq \frac{1000*10^{6}}{70}$
thus, $freq_{clock} \leq 14.2857 MHz$