Question 599450
Distance is equal to velocity multiplied by time   D=v*t

The distance traveled by the second biker will equal the distance traveled by the first at some point in time.

hence D1=first cyclist
                    v=6(m/h)
                    t=(3+x)h
            D2=second cyclist
                    v=10(m/h)
                    t=xh

we know D1 is 3 hours ahead and D2 has to travel (x) amount of time to catch up. 

so

D1=D2

6(3+x)=10x

18+6x=10x

18=4x

x=4.5 hours