If the question is only asking you to solve for t:

Step 1:

Step 2:

Step 3:

Step 4:

And you're done.

As for writing your math nicely, this forum uses LaTeX. There are plenty of guides for it online. To write your initial problem you would type: y = \frac{at^2}{2} with [ tex] before your equation and [ /tex] after it (minus the spaces).