Draw a picture, as you should do with all problems. Now, when the tire travels x inches forward, it means the circumference has moved along x inches. This means, that if the stone started at the bottom, it has now moved backwards by an angle θ relative to a vertical through the axis. The angle subtends a circumference of length x. The radius of the wheel is 2.5π inches. θ = arc length/radius = x/2.5π

Now, the pebble starts 5π inches to the right of the left most point. After the wheel goes forward x inches, the pebble moves back θ radians, so the distance backwards it moves is 5πsinθ inches (look at your diagram, draw a right angled triangle with hypotenuse equal to the radius of the wheel, when the stone has moved back by θ radians). So now the distance the stone is from the left most point, after it has moved forward x inches, is:

5π - 5πsinθ = 5π(1 - sinθ)

This is the distance L and we know θ = x/2.5π, so:

L = 5π[1 - sin(x/2.5π)]

I am sorry, I just can't seem to get this all the same font and size