You'll need to use kinetic equations here (as opposed to the questions you solved previously were you could use kinematic equations).
Start with the torque balance equation (i.e. the angular equivalent of Newton's 2nd law):
in which is resultant torque, is mass moment of inertia, and is angular acceleration/retardation.
is the variable that you want to solve this equation for. is known, but you still need to find the value of .
To find apply the angular momentum equation to the test data that was given: