Bicycle Wheel Radius Calculation

[LTZach]

Homework Statement

A bicycle wheel has an angular acceleration of 1.4 rad/s^2. If a point on its rim has a tangential acceleration of 49 cm/s^2, what is the radius of the wheel?
Answer in units of m.

Homework Equations

a_c=r*a

The Attempt at a Solution

49 cm/s^2=.49m/s^2
r=ac/a=2.8571

But the answer is wrong, where did I mess up?

 

[ehild]

Just to make things clear, what is a_c and what is a?

ehild