Moderate# Moons Collide: New Circular Orbit?

APPHMC-FSPIUR

Two identical, small moons are traveling in elliptical orbits around a planet (mass $M$, radius $R$). They crash into each other and stick together (losing no mass) and then settle into a stable circular orbit of radius $r$ where $r>>R.$

If the planets originally each have a mass of $m$, what is the orbital speed of the new, circular orbit?