20=3m+5m what does m equal?

Can someone check if i did this right since i really want to make sure it’s correct. if you do you so much

Randy and trey take turns cleaning offices on the weekends. it takes randy at most 4 hours to clean the offices. it takes trey at most 6 hours to clean the offices. what is the greatest amount of time it would take them to clean the offices together? 2.4 hours 2.5 hours

If f(x)=3 over x+2 - square root x-3 then f(7)=