Circumference of the wheel = 0.52 x pi = 1.633 metres (need the distance in metres for the later calculations)
This passes 3 times per second so the pebble travels 1.633 x 3 metres every second = 4.899 m/s
so the pebbles speed is 4.899 m/s
Now I am assuming this is a circular motion problem, so the acceleration the question is looking for is the acceleration due to the circular motion
This is given by acceleration = speed x speed /radius
a = 4.899 x 4.899 / 0.26 note that 0.26 is the radius which is half the diameter
a = 92.308 m/s/s the direction of the pebbles acceleration is towards the centre of the wheel.
I am ignoring any acceleration due to gravity.