Can someone explain how this works

Can someone me out here and the tell me the greatest common factor

What is the missing statement in step 4? ? rts ? ? vtu and ? rtu ? ? vts ? rts ? ? rvs and ? rtu ? ? stv ? vrs ? ? vru and ? usr ? ? usv ? vur ? ? vus and ? uvs ? ? sru

Ahousekeeper mixed 5 gallons of blue paint with every 9 gallon of yellow paint in order to make a green paint which ratio of gallon of blue paint to gallon of yellow paint will make the same shade of green paint