firstly your conversion of atm to pascals is wrong....cos 5.4 atm = 547 155 pascals
secondly p= force/area
area = 2.54X10^-4 m2
thus force = area X pressure = 139 N
Basically the penny will begin to slip when the centrifugal force is greater than the frictional force.
Your frictional force is Umg where U is the coefficient of friction(0.5) in this case.
Ur centrifugal force will be mw^2r where w is the angular velocity.
solve for Umg=mw^2r first to get w...