Finding Tangential Acceleration

[GingerBread27]

The flywheel of a steam engine runs with a constant angular speed of 425 rev/min. When steam is shut off, the friction of the bearings stops the wheel in 1.7 h.

At the instant the flywheel is turning at 75 rev/min, what is the tangential component of the linear acceleration of a flywheel particle that is 50 cm from the axis of rotation?

I know the equation to use is At=(alpha)r, where alpha is dw/dt. In this case I'm not sure how to compute the alpha and I'm sure it's just a simple point that I am missing.

[Spectre5]

first change the 1.7 hours to seconds, then the 425 rev/min to rad/s

then:

\alpha=\frac{d\omega}{dt}

\alpha dt=d\omega

\int_{0}^{t}\alpha dt=\int_{\omega}^{0}d\omega

[Doc Al]

In this case I'm not sure how to compute the alpha and I'm sure it's just a simple point that I am missing.
Use simple kinematics to find alpha: \alpha = \Delta \omega/\Delta t. (Take care with your units, as Spectre5 advises.)