Supermassive black holes, found in every massive galaxy, are now known to be key players in the evolution of galaxies. As galaxies merge, pairs of black holes can form and eventually lead to binary black holes emitting gravitational waves detectable by current pulsar timing arrays and future space-based observatories. There are discrepancies between current pulsar timing array data and our current understanding of the coevolution of galaxies and black holes. How black hole masses evolve relative to their host galaxies is unknown and may solve this discrepancy. In this talk, I will give a brief overview of how black hole masses are measured and how the masses relate to their host galaxies and what we still need to know. I will present recent (and still in progress) results of my group's work on looking at different black-hole-mass--galaxy scaling relations predict different results for gravitational wave background measurements, potentially solving the discrepancy. I will also discuss how X-ray and optical observations can break degeneracies in our understanding of how black holes pair up and merge. Together, these combined gravitational-wave and electromagnetic observations can bring a better understanding than they can individually.