Controlling a ring oscillator with a signal
I assume you've had enough time to think about this (and I also don't have enough karma to make comments) so I'll give you one possible solution, though it's definitely not the only one. Replace the 3rd inverter with a NAND gate, where one of the inputs is x and the other input is the output from the second inverter. When x is 0, the output is forced to 1 and the whole thing stops oscillating. When x is 1, the output of the NAND gate is the opposite of the output of the second oscillator.

You can see the truth table for a NAND gate here in case you don't get it. http://hyperphysics.phy-astr.gsu.edu/hbase/Electronic/nand.html