Question 31758
Let's look at our data first:
David:

His Distance = 945
His Time = x
His Speed = z

His Time = His Distance divided by His Speed
His Time = {{{945/z}}}

John:

His Distance = 135
His Time = Four less than David
His Speed = Three Times that of David

His Time = His Distance divided by His Speed
His Time = {{{135/Three Times that of David}}}
His Time = {{{135/3z}}}



John's Time was 4 hrs less than David. Therefore:

{{{135/3z}}}+4 = {{{945/z}}}

I'm sure you can solve this equation. I'll let you finish it off!