You have parts (a) through (e) done correctly. For (f), you are correct that you need to solve for t:
0 = 5015t+t^2
What do you get for t? Hint  this is a quadratic equation, so you can either factor the equation or use the quadratic formula to solve for t. You will find that there are actually two solutions.
As for (g), you have the right equation, but I think they want you to include the actual values of the coefficients Vo and a.
