Oh! You want some help. Please show some work next time, or at least your best guess.
For part iii. You know the speed of the block up the incline from part ii. Now this is just a motion in 1-D problem. You have the initial speed of the block (from part ii) and you are looking for the time such that the block comes to rest on the incline. The only thing you need is an acceleration, and you can find by finding the component of the weight down the incline. (Divide by m and you get the acceleration on the block.)
Give it a try from here and let us know how you are doing on it.
i use V=U+at as we dont know the distance covered so this is the only equation without distance. since v=0 and a=g the final velocity of part (ii) is the initial velocity of part (iii) . the problem is i get the negative answer as -U=at now can u help
Notice that this last line is pointing down the incline. This is the weight component in the direction of the incline. Now. The angle of the incline is the same as the angle between the weight vector and the line pointing in the opposite direction of the Normal line.
I have included the diagram which should (hopefully!) make the text make some more sense.
So assuming you've got the acceleration down the slope...Call up the slope positive. So your initial speed is positive (up the slope) and the acceleration is negative (down the slope.) You've got the initial speed, the final speed (0 m/s) and the acceleration. You are looking for the time where the object stops. What equation has all of these? Looks to me like you need .