Let
x be the speed of the slower train
x + 21 be the speed of the faster train
They meet in 6 hours which means that their speed is
[tex]\frac{906}{6}=151[/tex]The sum of their therefore is
[tex]\begin{gathered} x+(x+21)=151 \\ 2x+21=151 \\ 2x=151-21 \\ 2x=130 \\ \frac{2x}{2}=\frac{130}{2} \\ x=65\frac{\operatorname{mi}}{\operatorname{h}}\text{ (speed of slower train)} \\ \\ x+21=65+21=86\frac{\operatorname{mi}}{\operatorname{h}}\text{ (speed of faster train)} \end{gathered}[/tex]