Question 529922
We could try using algebra:

Let the speed of car be x
Then the speed of bus is x-21

{{{4x=6(x-21)}}} ---The speed of the car is x, it traveled 4 hours, so 4x represents the total distance, the bus uses 6 hours with a speed of x-21 so it also represents the distance
{{{4x=6x-126}}}
{{{-2x=-126}}}
{{{x=63(mph)}}}

 The speed of bus
=63-21
=42(mph)