w = w0 + a*t
theta = w0*t + 1/2*a*t^2
where w is the angular speed, w0 is the initial angular speed, and a is the angular acceleration, and theta is the angle turned in radians, so
3.14*10^4 = 1.2*10^4 + a *t
1.92*10^4 = 1.2*10^4*t + .5*a*t^2
Solve this system of equations using algebra for a and t
Then, use
7.85*10^4 = 1.2*10^4 + a*t,
where you know a from before, and you now solve for time