They were discovered by way of the Hubble Space Telescope and were found at the centers of other galaxies.
The smaller of the two was found at the center of galaxy NGC 3842 in the consolation Leo and has an estimated distance of about 331 million light-years. It mass is that 9.7 billion Suns but compared to other one it is actually small.
The larger one and undisputed champion of the suppermassive black holes was found at the center of galaxy NGC 4889 in the Coma constellation. This monster of a black hole has a the mass of a whopping 21 billion Suns.
Discoveries like these are show that the universe sill has many mysteries hiding in the depths of space. It needs to be noted that suppermassive black holes like this are so large that planets and even stars could fall through the event horizon intact.