Help me to figure out how this equation is derived

[physiker99]

Hello, I just couldn't understand how solver comes up with the equation at line 5 of the part b. (equates t (time) to other variables). I think the first 4 lines are enough to figure out this equation so i am not posting the question. i will be grateful if you'd help me.http://www.cramster.com/Answer-Board/Image/200863929416334808218196100003057.jpg

 

[Avodyne]

He uses $$R\omega=at$$ (line 3) in line 2, and solves for t.